Heterogeneous catalysts

ABSTRACT

Heterogeneous catalysts with optional dopants are provided. The catalysts are useful in a variety of catalytic reactions, for example, the oxidative coupling of methane to C 2+  hydrocarbons. Related methods for use and manufacture of the same are also disclosed.

BACKGROUND

Technical Field

This invention is generally related to novel heterogeneous catalysts and, more specifically, to heterogeneous catalysts useful in a variety of catalytic reactions, such as the oxidative coupling of methane to C₂₊ hydrocarbons.

Description of the Related Art

Catalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a catalyst. Positive catalysts increase the speed of a chemical reaction, while negative catalysts slow it down. Substances that increase the activity of a catalyst are referred to as promoters or activators, and substances that deactivate a catalyst are referred to as catalytic poisons or deactivators. Unlike other reagents, a catalyst is not consumed by the chemical reaction, but instead participates in multiple chemical transformations. In the case of positive catalysts, the catalytic reaction generally has a lower rate-limiting free energy change to the transition state than the corresponding uncatalyzed reaction, resulting in an increased reaction rate at the same temperature. Thus, at a given temperature, a positive catalyst tends to increase the yield of desired product while decreasing the yield of undesired side products. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated or destroyed by secondary processes, resulting in loss of catalytic activity.

Catalysts are generally characterized as either heterogeneous or homogeneous. Heterogeneous catalysts exist in a different phase than the reactants (e.g., a solid metal catalyst and gas phase reactants), and the catalytic reaction generally occurs on the surface of the heterogeneous catalyst. Thus, for the catalytic reaction to occur, the reactants must diffuse to and/or adsorb onto the catalyst surface. This transport and adsorption of reactants is often the rate limiting step in a heterogeneous catalysis reaction. Heterogeneous catalysts are also generally easily separable from the reaction mixture by common techniques such as filtration or distillation.

In contrast to a heterogeneous catalyst, a homogenous catalyst exists in the same phase as the reactants (e.g., a soluble organometallic catalyst and solvent-dissolved reactants). Accordingly, reactions catalyzed by a homogeneous catalyst are controlled by different kinetics than a heterogeneously catalyzed reaction. In addition, homogeneous catalysts can be difficult to separate from the reaction mixture.

While catalysis is involved in a number of different technologies, one particular area of importance is the petrochemical industry. At the foundation of the modern petrochemical industry is the energy-intensive endothermic steam cracking of crude oil. Cracking is used to produce nearly all the fundamental chemical intermediates in use today. The amount of oil used for cracking and the volume of greenhouse gases (GHG) emitted in the process are quite large: cracking consumes nearly 10% of the total oil extracted globally and produces 200M metric tons of CO₂ equivalent every year (Ren, T, Patel, M. Res. Conserv. Recycl. 53:513, 2009). There remains a significant need in this field for new technology directed to the conversion of unreactive petrochemical feedstocks (e.g., paraffins, methane, ethane, etc.) into reactive chemical intermediates (e.g., olefins), particularly with regard to highly selective heterogeneous catalysts for the direct oxidation of hydrocarbons.

While there are multistep paths to convert methane to certain specific chemicals using first; high temperature steam reforming to syngas (a mixture of H₂ and CO), followed by stoichiometry adjustment and conversion to either methanol or, via the Fischer-Tropsch (F-T) synthesis, to liquid hydrocarbon fuels such as diesel or gasoline, this does not allow for the formation of certain high value chemical intermediates. This multi-step indirect method also requires a large capital investment in facilities and is expensive to operate, in part due to the energy intensive endothermic reforming step. For instance, in methane reforming, nearly 40% of methane is consumed as fuel for the reaction. It is also inefficient in that a substantial part of the carbon fed into the process ends up as the GHG CO₂, both directly from the reaction and indirectly by burning fossil fuels to heat the reaction. Thus, to better exploit the natural gas resource, direct methods that are more efficient, economical and environmentally responsible are required.

One of the reactions for direct natural gas activation and its conversion into a useful high value chemical, is the oxidative coupling of methane (“OCM”) to ethylene: 2CH₄+O₂→C₂H₄+2H₂O. See, e.g., Zhang, Q., Journal of Natural Gas Chem., 12:81, 2003; Olah, G. “Hydrocarbon Chemistry”, Ed. 2, John Wiley & Sons (2003). This reaction is exothermic (ΔH=−67 kcals/mole) and has typically been shown to occur at very high temperatures (>700° C.). Although the detailed reaction mechanism is not fully characterized, experimental evidence suggests that free radical chemistry is involved. (Lunsford, J. Chem. Soc., Chem. Comm., 1991; H. Lunsford, Angew. Chem., Int. Ed. Engl., 34:970, 1995). In the reaction, methane (CH₄) is activated on the catalyst surface, forming methyl radicals which then couple in the gas phase to form ethane (C₂H₆), followed by dehydrogenation to ethylene (C₂H₄). Several catalysts, with and without dopants, have shown activity for OCM; however, none of these catalysts have shown sufficient activity for commercial-scale implementation of OCM.

Since the OCM reaction was first reported over thirty years ago, it has been the target of intense scientific and commercial interest, but the fundamental limitations of the conventional approach to C—H bond activation appear to limit the yield of this attractive reaction. Specifically, numerous publications from industrial and academic labs have consistently demonstrated characteristic performance of high selectivity at low conversion of methane, or low selectivity at high conversion (J. A. Labinger, Cat. Lett., 1:371, 1988). Limited by this conversion/selectivity threshold, no OCM catalyst has been able to exceed 20-25% combined C₂ yield (i.e., ethane and ethylene), and more importantly, all such reported yields operate at extremely high temperatures (>800 C).

In this regard, it is believed that the low yield of desired products (i.e., C₂H₄ and C₂H₆) is caused by the unique homogeneous/heterogeneous nature of the reaction. Specifically, due to the high reaction temperature, a majority of methyl radicals escape the catalyst surface and enter the gas phase. There, in the presence of oxygen and hydrogen, multiple side reactions are known to take place (J. A. Labinger, Cat. Lett., 1:371, 1988). The non-selective over-oxidation of hydrocarbons to CO and CO₂ (e.g., complete oxidation) is the principal competing fast side reaction. Other undesirable products (e.g., methanol, formaldehyde) have also been observed and rapidly react to form CO and CO₂.

In order to result in a commercially viable OCM process, a catalyst optimized for the activation of the C—H bond of methane at lower temperatures (e.g., 500-800° C.) higher activities, and higher pressures are required. While the above discussion has focused on the OCM reaction, numerous other catalytic reactions (as discussed in greater detail below) would significantly benefit from catalytic optimization. Accordingly, there remains a need in the art for improved catalysts and, more specifically, catalysts for improving the yield, selectivity and conversion of, for example, the OCM reaction and other catalyzed reactions. The present invention fulfills these needs and provides further related advantages.

BRIEF SUMMARY

In brief, heterogeneous metal oxide catalysts and related methods are disclosed. For example, catalysts comprising oxides of one or more lanthanide elements and optional dopants are provided in various embodiments. The disclosed catalysts find utility in any number of catalytic reactions, for example in the OCM reaction. In some embodiments, the catalysts are advantageously doped with one or more doping elements. The doping elements may be promoters such that the catalyst comprises an improved catalytic activity. For example, in certain embodiments, the catalytic activity is such that the C2+ selectivity is 50% or greater and the methane conversion is 20% or greater when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 850° C. or less, 800° C. or less, for example 750° C. or less or 700° C. or less.

Methods for use of the disclosed catalysts in catalytic reactions, for example OCM, are also provided. Furthermore, the present disclosure also provides for the preparation of downstream products of ethylene, wherein the ethylene has been prepared via a reaction employing a catalyst disclosed herein.

These and other aspects of the invention will be apparent upon reference to the following detailed description. To this end, various references are set forth herein which describe in more detail certain background information, procedures, compounds and/or compositions, and are each hereby incorporated by reference in their entirety.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, the sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not intended to convey any information regarding the actual shape of the particular elements, and have been selected solely for ease of recognition in the drawings.

FIG. 1 schematically depicts a first part of an OCM reaction at the surface of a metal oxide catalyst.

FIG. 2 shows a method for catalyst screening.

FIG. 3 schematically depicts a carbon dioxide reforming reaction on a catalytic surface.

FIG. 4 is a flow chart for data collection and processing in evaluating catalytic performance.

FIG. 5 is a chart showing various downstream products of ethylene.

FIG. 6 shows an OCM and ethylene oligomerization module.

FIGS. 7A and 7B show scanning electron micrographs of representative catalysts.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” Further, headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Also, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.

As discussed above, heterogeneous catalysis takes place between at least two phases. Generally, the catalyst is a solid, the reactants are gases or liquids and the products are gases or liquids. While not wishing to be bound by theory, it is believed that a heterogeneous catalyst provides a surface that has multiple active sites for adsorption of one more gas or liquid reactants. Once adsorbed, certain bonds within the reactant molecules are weakened and dissociate, creating reactive fragments of the reactants, e.g., in free radical forms. One or more products are generated as new bonds between the resulting reactive fragments form, in part, due to their proximity to each other on the catalytic surface.

As an example, FIG. 1 shows schematically a theoretical representation of the first part of an OCM reaction that takes place on the surface of an exemplary metal oxide catalyst 10 which is followed by methyl radical coupling in the gas phase. A crystal lattice structure of metal atoms 14 and oxygen atoms 20 are shown, with an optional dopant 24 incorporated into the lattice structure. In this reaction, a methane molecule 28 comes into contact with an active site (e.g., surface oxygen 30) and becomes activated when a hydrogen atom 34 dissociates from the methane molecule 28. As a result, a methyl radical 40 is generated on or near the catalytic surface. Two methyl radicals thus generated can couple in the gas phase to create ethane and/or ethylene, which are collectively referred to as the “C2” coupling products.

It is generally recognized that the catalytic properties of a catalyst strongly correlate to its surface morphology. Typically, the surface morphology can be defined by geometric parameters such as: (1) the number of surface atoms (e.g., the surface oxygen of FIG. 1) that coordinate to the reactant; and (2) the degree of coordinative unsaturation of the surface atoms, which is the coordination number of the surface atoms with their neighboring atoms. For example, the reactivity of a surface atom decreases with decreasing coordinative unsaturation. For example, for the dense surfaces of a face-centered crystal, a surface atom with 9 surface atom neighbors will have a different reactivity than one with 8 neighbors. Additional surface characteristics that may contribute to the catalytic properties include, for example, crystal dimensions, lattice distortion, surface reconstructions, defects, grain boundaries, and the like. See, e.g., Van Santen R. A. et al New Trends in Materials Chemistry 345-363 (1997).

Advantageously, embodiments of the catalysts disclosed herein and methods of producing the same have general applicability to a wide variety of heterogeneous catalyses, including without limitation: oxidative coupling of methane (e.g., FIG. 1), oxidative dehydrogenation of alkanes to their corresponding alkenes, selective oxidation of alkanes to alkenes and alkynes, oxidation of carbon monoxide, dry reforming of methane, selective oxidation of aromatics, Fischer-Tropsch reaction, hydrocarbon cracking, combustion of hydrocarbons and the like.

FIG. 2 schematically shows an exemplary high throughput work flow for generating libraries of diverse catalysts and screening for their catalytic properties. An initial phase of the work flow involves a primary screening, which is designed to broadly and efficiently screen a large and diverse set of catalysts that logically could perform the desired catalytic transformation. For example, certain doped metal oxides (e.g., Mn, Mg, W, etc.) are known catalysts for the OCM reaction. Therefore, catalysts of various metal oxide compositions comprising various dopants can be prepared and evaluated for their catalytic performances in an OCM reaction.

More specifically, the work flow 100 begins with designing synthetic experiments for making various metal oxide compositions (block 110). The synthesis, subsequent treatments and screenings can be manual or automated. As will be discussed in more detail herein, by varying the synthetic conditions, catalysts can be prepared with various surface morphologies and/or compositions in respective microwells (block 114). The catalysts are subsequently calcined and optionally doped (block 120). As discussed in more detail herein, the optional dopants may be incorporated during preparation of the catalysts (114) or as a separate step after preparation of the catalysts and before or after calcination. Optionally, the catalysts are further mixed with a catalyst support (block 122). As with the dopant, the optional support can also be incorporated at any point in the preparation of the catalysts, and its incorporation is not limited by the depicted order in FIG. 2.

Beyond the optional support step, all subsequent steps are carried out in a “wafer” format, in which catalysts are deposited in a quartz wafer that has been etched to create an ordered array of microwells. Each microwell is a self-contained reactor, in which independently variable processing conditions can be designed to include, without limitation, respective choices of elemental compositions, catalyst support, reaction precursors, templates, reaction durations, pH values, temperatures, ratio between reactants, gas flows, and calcining conditions (block 124). Due to design constraints of some wafers, in some embodiments calcining and other temperature variables are identical in all microwells. A wafer map 130 can be created to correlate the processing conditions to the catalyst in each microwell. A library of diverse catalysts can be generated in which each library member corresponds to a particular set of processing conditions and corresponding compositional and/or morphological characteristics.

Catalysts obtained under various synthetic conditions and doping compositions are thereafter deposited in respective microwells of a wafer (140) for evaluating their respective catalytic properties in a given reaction (blocks 132 and 134). The catalytic performance of each library member can be screened serially by several known primary screening technologies, including scanning mass spectroscopy (SMS) (Symyx Technologies Inc., Santa Clara, Calif.). The screening process is fully automated, and the SMS tool can determine if a catalyst is catalytically active or not, as well as its relative strength as a catalyst at a particular temperature. Typically, the wafer is placed on a motion control stage capable of positioning a single well below a probe that flows the feed of the starting material over the catalyst surface and removes reaction products to a mass spectrometer and/or other detector technologies (blocks 134 and 140). The individual catalyst is heated to a preset reaction temperature, e.g., using a CO₂ IR laser from the backside of the quartz wafer and an IR camera to monitor temperature and a preset mixture of reactant gases. The SMS tool collects data with regard to the consumption of the reactant(s) and the generation of the product(s) of the catalytic reaction in each well (block 144), and at each temperature and flow rate.

The SMS data obtained as described above provide information on relative catalytic properties among all the library members (block 150). In order to obtain more quantitative data on the catalytic properties of the catalysts, possible hits that meet certain criteria are subjected to a secondary screening (block 154). Typically, secondary screening technologies include a single, or alternatively multiple channel fixed-bed or fluidized bed reactors (as described in more detail herein). In parallel reactor systems or multi-channel fixed-bed reactor system, a single feed system supplies reactants to a set of flow restrictors. The flow restrictors divide the flows evenly among parallel reactors. Care is taken to achieve uniform reaction temperature between the reactors such that the various catalysts can be differentiated solely based on their catalytic performances. The secondary screening allows for accurate determination of catalytic properties such as selectivity, yield and conversion (block 160). These results serve as a feedback for designing further catalyst libraries.

Secondary screening is also schematically depicted in FIG. 4, which depicts a flow chart for data collection and processing in evaluating catalytic performance of catalysts according to the invention. Additional description of SMS tools in a combinatorial approach for discovering catalysts can be found in, e.g., Bergh, S. et al. Topics in Catalysts 23:1-4, 2003.

Thus, in accordance with various embodiments described herein, compositional and morphologically diverse catalysts can be rationally synthesized to meet catalytic performance criteria. These and other aspects of the present disclosure are described in more detail below.

Definitions

As used herein, and unless the context dictates otherwise, the following terms have the meanings as specified below.

“Catalyst” means a substance which alters the rate of a chemical reaction. A catalyst may either increase the chemical reaction rate (i.e., a “positive catalyst”) or decrease the reaction rate (i.e., a “negative catalyst”). Catalysts participate in a reaction in a cyclic fashion such that the catalyst is cyclically regenerated. “Catalytic” means having the properties of a catalyst.

“Turnover number” is a measure of the number of reactant molecules a catalyst can convert to product molecules per unit time.

“Active” or “catalytically active” refers to a catalyst which has substantial activity in the reaction of interest. For example, in some embodiments a catalyst which is OCM active (i.e., has activity in the OCM reaction) has a C2+ selectivity of 5% or more and/or a methane conversion of 5% or more when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

“Inactive” or “catalytically inactive” refers to a catalyst which does not have substantial activity in the reaction of interest. For example, in some embodiments a catalyst which is OCM inactive has a C2+ selectivity of less than 5% and/or a methane conversion of less than 5% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

“Activation temperature” refers to the temperature at which a catalyst becomes catalytically active.

“OCM activity” refers to the ability of a catalyst to catalyze the OCM reaction.

A catalyst having “high OCM activity” refers to a catalyst having a C2+ selectivity of 50% or more and/or a methane conversion of 20% or more when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a specific temperature, for example 750° C. or less.

A catalyst having “moderate OCM activity” refers to a catalyst having a C2+ selectivity of about 20-50% and/or a methane conversion of about 10-20% or more when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

A catalyst having “low OCM activity” refers to a catalyst having a C2+ selectivity of about 5-20% and/or a methane conversion of about 5-10% or more when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

“Base material” refers to the major component of a catalyst. For example a mixed oxide of manganese and magnesium which is doped with lithium and/or boron comprises a manganese/magnesium oxide base material.

“Dopant” or “doping agent” or “doping element” is a chemical element (or in some embodiments a compound) which is added to or incorporated within a catalyst base material to optimize catalytic performance (e.g., increase or decrease catalytic activity). As compared to the undoped catalyst, a doped catalyst may increase or decrease the selectivity, conversion, and/or yield of a reaction catalyzed by the catalyst. Dopants which increase catalytic activity are referred to as “promoters” while dopants which decrease catalytic activity are referred to as “poisons”. For example, a promoter with respect to a certain reaction (e.g., OCM) refers to a dopant which increases the catalytic activity of the catalyst (relative to undoped catalyst) in the reaction. In some embodiments, an increase in catalytic activity is evidenced by any one or more of: an increase in yield (e.g., C2+ yield), an increase in selectivity (e.g., C2+ selectivity), and increase in conversion (e.g., methane conversion) or a decrease in the temperature required to maintain the same yield, selectivity and/or conversion. The dopant may be present in the catalyst in any form and may be derived from any suitable source of the element (e.g., elemental form, chlorides, bromides, iodides, nitrates, oxynitrates, oxyhalides, acetates, formates, hydroxides, carbonates, phosphates, sulfates, alkoxides, oxides and the like.)

“Atomic percent” (at % or at/at) or “atomic ratio” when used in the context of catalyst dopants refers to the ratio of the total number of dopant atoms to the total number of non-oxygen atoms in the base material. For example, the atomic percent of dopant in a lithium doped Mg₆MnO₈ catalyst is determined by calculating the total number of lithium atoms and dividing by the sum of the total number of magnesium and manganese atoms and multiplying by 100 (i.e., atomic percent of dopant=[Li atoms/(Mg atoms+Mn atoms)]×100).

“Weight percent” (wt/wt)” when used in the context of catalyst dopants refers to the ratio of the total weight of dopant to the total combined weight of the dopant and the catalyst. For example, the weight percent of dopant in a lithium doped Mg₆MnO₈ catalyst is determined by calculating the total weight of lithium and dividing by the sum of the total combined weight of lithium and Mg₆MnO₈ and multiplying by 100 (i.e., weight percent of dopant=[Li weight/(Li weight+Mg₆MnO₈ weight)]×100).

“Group 1” elements include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

“Group 2” elements include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

“Group 3” elements include scandium (Sc) and yttrium (Y).

“Group 4” elements include titanium (Ti), zirconium (Zr), hafnium (Hf), and rutherfordium (Rf).

“Group 5” elements include vanadium (V), niobium (Nb), tantalum (Ta), and dubnium (Db).

“Group 6” elements include chromium (Cr), molybdenum (Mo), tungsten (W), and seaborgium (Sg).

“Group 7” elements include manganese (Mn), technetium (Tc), rhenium (Re), and bohrium (Bh).

“Group 8” elements include iron (Fe), ruthenium (Ru), osmium (Os), and hassium (Hs).

“Group 9” elements include cobalt (Co), rhodium (Rh), iridium (Ir), and meitnerium (Mt).

“Group 10” elements include nickel (Ni), palladium (Pd), platinum (Pt) and darmistadium (Ds).

“Group 11” elements include copper (Cu), silver (Ag), gold (Au), and roentgenium (Rg).

“Group 12” elements include zinc (Zn), cadmium (Cd), mercury (Hg), and copernicium (Cn).

“Group 13” elements include boron (B), aluminum (Al), gallium (Ga), indium (In) and thallium (Tl).

“Group 16” elements include oxygen (O), sulfur (S), selenium (Se), tellurium (Te) and polonium (Po).

“Lanthanides” include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

“Actinides” include actinium (Ac), thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (Cf), einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No), and lawrencium (Lr).

“Rare earth elements” include the lanthanides, actinides and Group 3.

“Metal element” or “metal” is any element, except hydrogen, selected from Groups 1 through 12, lanthanides, actinides, aluminum (Al), gallium (Ga), indium (In), tin (Sn), thallium (Tl), lead (Pb), and bismuth (Bi). Metal elements include metal elements in their elemental form as well as metal elements in an oxidized or reduced state, for example, when a metal element is combined with other elements in the form of compounds comprising metal elements. For example, metal elements can be in the form of hydrates, salts, oxides, as well as various polymorphs thereof, and the like.

“Semi-metal element” refers to an element selected from boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), and polonium (Po).

“Non-metal element” refers to an element selected from carbon (C), nitrogen (N), oxygen (O), fluorine (F), phosphorus (P), sulfur (S), chlorine (Cl), selenium (Se), bromine (Br), iodine (I), and astatine (At).

“C2” refers to a hydrocarbon (i.e., compound consisting of carbon and hydrogen atoms) having only two carbon atoms, for example ethane and ethylene. Similarly, “C3” refers to a hydrocarbon having only 3 carbon atoms, for example propane and propylene.

“Conversion” means the mole fraction (i.e., percent) of a reactant converted to a product or products.

“Selectivity” refers to the percent of converted reactant that went to a specified product, e.g., C2 selectivity is the % of converted methane that formed ethane and ethylene, C3 selectivity is the % of converted methane that formed propane and propylene, C2+ selectivity is the % of converted methane that formed ethane, ethylene, propane, propylene and other higher hydrocarbons (hydrocarbons comprising 2 or more carbons). CO selectivity is the % of converted methane that formed CO.

“Yield” is a measure of (e.g., percent) of product obtained relative to the theoretical maximum product obtainable. Yield is calculated by dividing the amount of the obtained product in moles by the theoretical yield in moles. Percent yield is calculated by multiplying this value by 100. C2 yield is defined as the sum of the ethane and ethylene molar flow at the reactor outlet multiplied by two and divided by the inlet methane molar flow. C3 yield is defined as the sum of propane and propylene molar flow at the reactor outlet multiplied by three and divided by the inlet methane molar flow. C2+ yield is the sum of the C2 yield and C3 yield and the yield of other higher hydrocarbons should they be present in any measurable quantities. Yield is also calculable by multiplying the methane conversion by the relevant selectivity, e.g., C2 yield is equal to the methane conversion times the C2 selectivity. C2+ yield is equal to the methane conversion times the C2+ selectivity.

“Bulk catalyst” or “bulk material” means a catalyst prepared by traditional techniques, for example by milling or grinding large catalyst particles to obtain smaller/higher surface area catalyst particles.

“Nanostructured catalyst” means a catalyst having at least one dimension on the order of nanometers (e.g., between about 1 and 100 nanometers). Non-limiting examples of nanostructured catalysts include nanoparticle catalysts and nanowire catalysts.

“Nanoparticle” means a particle having at least one diameter on the order of nanometers (e.g., between about 1 and 100 nanometers).

“Nanowire” means a wire-like structure having at least one diameter on the order of nanometers (e.g., between about 1 and 100 nanometers) and an aspect ratio greater than 10:1. The “aspect ratio” of a nanowire is the ratio of the actual length (L) of the nanowire to the diameter (D) of the nanowire. Aspect ratio is expressed as L:D. Exemplary nanowires are known in the art and described in more detail in co-pending U.S. application Ser. No. 13/115,082 (U.S. Pub. No. 2012/0041246), Ser. No. 13/689,514 (U.S. Pub. No. 2013/0158322) and Ser. No. 13/689,611 (U.S. Pub. No. 2013/0165728), the full disclosures of which are hereby incorporated by reference in their entireties for all purposes.

“Polycrystalline nanowire” means a nanowire having multiple crystal domains. Polycrystalline nanowires generally have different morphologies (e.g., bent vs. straight) as compared to the corresponding “single-crystalline” nanowires.

“Crystal domain” means a continuous region over which a substance is crystalline.

“Single-crystalline nanowire” means a nanowire having a single crystal domain.

“Effective length” of a nanowire means the shortest distance between the two distal ends of a nanowire as measured by transmission electron microscopy (TEM) in bright field mode at 5 keV. “Average effective length” refers to the average of the effective lengths of individual nanowires within a plurality of nanowires.

“Actual length” of a nanowire means the distance between the two distal ends of a nanowire as traced through the backbone of the nanowire as measured by TEM in bright field mode at 5 keV. “Average actual length” refers to the average of the actual lengths of individual nanowires within a plurality of nanowires.

The “diameter” of a nanowire is measured in an axis perpendicular to the axis of the nanowire's actual length (i.e., perpendicular to the nanowires backbone). The diameter of a nanowire will vary from narrow to wide as measured at different points along the nanowire backbone. As used herein, the diameter of a nanowire is the most prevalent (i.e., the mode) diameter.

The “ratio of effective length to actual length” is determined by dividing the effective length by the actual length. A nanowire having a “bent morphology” will have a ratio of effective length to actual length of less than one as described in more detail herein. A straight nanowire will have a ratio of effective length to actual length equal to one as described in more detail herein.

“Inorganic” means a substance comprising a metal element or semi-metal element. In certain embodiments, inorganic refers to a substance comprising a metal element. An inorganic compound can contain one or more metals in its elemental state, or more typically, a compound formed by a metal ion (M^(n+), wherein n 1, 2, 3, 4, 5, 6 or 7) and an anion (X^(m−), m is 1, 2, 3 or 4), which balance and neutralize the positive charges of the metal ion through electrostatic interactions. Non-limiting examples of inorganic compounds include oxides, hydroxides, halides, nitrates, sulfates, carbonates, phosphates, acetates, oxalates, and combinations thereof, of metal elements. Other non-limiting examples of inorganic compounds include Li₂CO₃, Li₂PO₄, LiOH, Li₂O, LiCl, LiBr, LiI, Li₂C₂O₄, Li₂SO₄, Na₂CO₃, Na₂PO₄, NaOH, Na₂O, NaCl, NaBr, NaI, Na₂C₂O₄, Na₂SO₄, K₂CO₃, K₂PO₄, KOH, K₂O, KCl, KBr, KI, K₂C₂O₄, K₂SO₄, Cs₂CO₃, CsPO₄, CsOH, CS₂O, CsCl, CsBr, CsI, CsC₂O₄, CsSO₄, Be(OH)₂, BeCO₃, BePO₄, BeO, BeCl₂, BeBr₂, BeI₂, BeC₂O₄, BeSO₄, Mg(OH)₂, MgCO₃, MgPO₄, MgO, MgCl₂, MgBr₂, MgI₂, MgC₂O₄, MgSO₄, Ca(OH)₂, CaO, CaCO₃, CaPO₄, CaCl₂, CaBr₂, CaI₂, Ca(OH)₂, CaC₂O₄, CaSO₄, Y₂O₃, Y₂(CO₃)₃, Y₂(PO₄)₃, Y(OH)₃, YCl₃, YBr₃, YI₃, Y₂(C₂O4)₃, Y₂(SO4)₃, Zr(OH)₄, Zr(CO₃)₂, Zr(PO₄)₂, ZrO(OH)₂, ZrO2, ZrCl₄, ZrBr₄, ZrI₄, Zr(C₂O₄)₂, Zr(SO₄)₂, Ti(OH)₄, TiO(OH)₂, Ti(CO₃)₂, Ti(PO₄)₂, TiO₂, TiCl₄, TiBr₄, TiI₄, Ti(C₂O₄)₂, Ti(SO₄)₂, BaO, Ba(OH)₂, BaCO₃, BaPO₄, BaCl₂, BaBr₂, BaI₂, BaC₂O₄, BaSO₄, La(OH)₃, La₂(CO₃)₃, La₂(PO₄)₃, La₂O₃, LaCl₃, LaBr₃, LaI₃, La₂(C₂O₄)₃, La₂(SO₄)₃, Ce(OH)₄, Ce(CO₃)₂, Ce(PO₄)₂, CeO₂, Ce₂O₃, CeCl₄, CeBr₄, CeI₄, Ce(C₂O₄)₂, Ce(SO₄)₂, ThO₂, Th(CO₃)₂, Th(PO₄)₂, ThCl₄, ThBr₄, ThI₄, Th(OH)₄, Th(C₂O₄)₂, Th(SO₄)₂, Sr(OH)₂, SrCO₃, SrPO₄, SrO, SrCl₂, SrBr₂, SrI₂, SrC₂O₄, SrSO₄, Sm₂O₃, Sm₂(CO₃)₃, Sm₂(PO₄)₃, SmCl₃, SmBr₃, SmI₃, Sm(OH)₃, Sm₂(CO3)₃, Sm₂(C₂O₃)₃, Sm₂(SO₄)₃, LiCa₂Bi₃O₄Cl₆, Na₂WO₄, K/SrCoO₃, K/Na/SrCoO₃, Li/SrCoO₃, SrCoO₃, molybdenum oxides, molybdenum hydroxides, molybdenum carbonates, molybdenum phosphates, molybdenum chlorides, molybdenum bromides, molybdenum iodides, molybdenum oxalates, molybdenum sulfates, manganese oxides, manganese chlorides, manganese bromides, manganese iodides, manganese hydroxides, manganese oxalates, manganese sulfates, manganese tungstates, vanadium oxides, vanadium carbonates, vanadium phosphates, vanadium chlorides, vanadium bromides, vanadium iodides, vanadium hydroxides, vanadium oxalates, vanadium sulfates, tungsten oxides, tungsten carbonates, tungsten phosphates, tungsten chlorides, tungsten bromides, tungsten iodides, tungsten hydroxides, tungsten oxalates, tungsten sulfates, neodymium oxides, neodymium carbonates, neodymium phosphates, neodymium chlorides, neodymium bromides, neodymium iodides, neodymium hydroxides, neodymium oxalates, neodymium sulfates, europium oxides, europium carbonates, europium phosphates, europium chlorides, europium bromides, europium iodides, europium hydroxides, europium oxalates, europium sulfates rhenium oxides, rhenium carbonates, rhenium phosphates, rhenium chlorides, rhenium bromides, rhenium iodides, rhenium hydroxides, rhenium oxalates, rhenium sulfates, chromium oxides, chromium carbonates, chromium phosphates, chromium chlorides, chromium bromides, chromium iodides, chromium hydroxides, chromium oxalates, chromium sulfates, potassium molybdenum oxides and the like.

“Salt” means a compound comprising negative and positive ions. Salts are generally comprised of cations and counter ions. Under appropriate conditions, e.g., the solution also comprises a template, the metal ion (M^(n+)) and the anion (X^(m−)) bind to the template to induce nucleation and growth of a nanowire of M_(m)X_(n) on the template. “Anion precursor” thus is a compound that comprises an anion and a cationic counter ion, which allows the anion (X^(m−)) to dissociate from the cationic counter ion in a solution.

“Oxide” generally refers to an oxidized element (i.e., oxidation state greater than 0). Oxides generally comprise oxygen. Examples of oxides include, but are not limited to oxidized metals such as, metal oxides (M_(x)O_(y)), metal oxyhalides (M_(x)O_(y)X_(z)), metal oxynitrates (M_(x)O_(y)(NO₃)_(z)), metal phosphates (M_(x)(PO₄)_(y)), metal oxycarbonates (M_(x)O_(y)(CO₃)_(z)), metal carbonates, metal oxyhydroxides (M_(x)O_(y)(OH)_(z)) and the like, wherein X is independently, at each occurrence, fluoro, chloro, bromo or iodo, and x, y and z are numbers from 1 to 100. For purpose of simplicity, the foregoing exemplary oxides are illustrated with one metal (M); however, it should be noted that in certain embodiments oxides will include more than one metal. The additional metal(s) may be present as part of the base material and/or as a doping element.

“Catalytic material” refers to a plurality of catalyst particles, which may optionally be combined with a support, diluent and/or binder.

“Catalyst form” or “catalytic form” refers to the physical shape of a catalytic material. For example, catalyst forms include catalysts in the shape of extrudates or pellets or disposed on various support structures, including honeycomb structures, grids, monoliths, and the like, as discussed in more detail below.

“Catalyst formulation” or “catalytic formulation” refers to the chemical composition of a catalytic material. For example, a catalyst formulation may include a catalyst and one or more support, diluent and/or binder materials.

An “extrudate” refers to a material (e.g., catalytic material) prepared by forcing a semisolid material comprising a catalyst through a die or opening of appropriate shape. Extrudates can be prepared in a variety of shapes and structures by common means known in the art.

A “formed aggregate” refers to an aggregation of catalyst material particles, either alone, or in conjunction with one or more other materials, e.g., catalyst materials, dopants, diluents, support materials, binders, etc., formed into a single particle. Formed aggregates include without limitation, extruded particles, termed “extrudates”, pressed or cast particles, e.g., pellets such as tablets, ovals, spherical particles, etc., coated particles, e.g., spray, immersion or pan coated particles, impregnated particles, e.g., monoliths, foils, foams, honeycombs, or the like. Formed aggregates may range in size from particles having individual cross sections in the micron range to cross sections in the millimeter range, to even larger particles such as monolithic formed aggregates, that may be on the order of centimeters or even meters in cross section.

A “pellet” or “pressed pellet” refers to a material (e.g., catalytic material) prepared by applying pressure to (i.e., compressing) a material comprising a catalyst into a desired shape. Pellets having various dimensions and shapes can be prepared according to common techniques in the art.

“Monolith” or “monolith support” is generally a structure formed from a single structural unit preferably having passages disposed through it in either an irregular or regular pattern with porous or non-porous walls separating adjacent passages. Examples of such monolithic supports include, e.g., ceramic or metal foam-like or porous structures. The single structural unit may be used in place of or in addition to conventional particulate or granular catalysts (e.g., pellets or extrudates). Examples of such irregular patterned monolith substrates include filters used for molten metals. Monoliths generally have a porous fraction, for example ranging from about 60% to 90%, and a flow resistance substantially less than the flow resistance of a packed bed of similar volume (e.g., about 10% to 30% of the flow resistance of a packed bed of similar volume). Examples of regular patterned substrates include monolith honeycomb supports used for purifying exhausts from motor vehicles and used in various chemical processes and ceramic foam structures having irregular passages. Many types of monolith support structures made from conventional refractory or ceramic materials such as alumina, zirconia, yttria, silicon carbide, and mixtures thereof, are well known and commercially available from, among others, Corning, Iac.; Vesuvius Hi-Tech Ceramics, Inc.; and Porvair Advanced Materials, Inc. and SiCAT (Sicatalyst.com). Monoliths include foams, honeycombs, foils, mesh, gauze and the like.

“Alkane” means a straight chain or branched, noncyclic or cyclic, saturated aliphatic hydrocarbon. Representative straight chain alkanes include methane, ethane, n-propane, n-butane, n-pentane, n-hexane, and the like; while branched alkanes include isopropane, sec-butane, isobutane, tert-butane, isopentane, and the like. Representative cyclic alkanes include cyclopropane, cyclobutane, cyclopentane, cyclohexane, and the like.

“Alkene” means a straight chain or branched, noncyclic or cyclic, unsaturated aliphatic hydrocarbon having at least one carbon-carbon double bond. Representative straight chain and branched alkenes include ethylene, propylene, 1-butene, 2-butene, isobutene, 1-pentene, 2-pentene, 3-methyl-1-butene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, and the like. Cyclic alkenes include cyclohexene and cyclopentene and the like.

“Alkyne” means a straight chain or branched, noncyclic or cyclic, unsaturated aliphatic hydrocarbon having at least one carbon-carbon triple bond. Representative straight chain and branched alkynes include acetylene, propyne, 1-butyne, 2-butyne, 1-pentyne, 2-pentyne, 3-methyl-1-butyne, and the like. Representative cyclic alkynes include cycloheptyne and the like.

“Alkyl,” “alkenyl” and “alkynyl” refer to an alkane, alkene or alkyne radical, respectively.

“Aromatic” means a cyclic moiety (e.g., carbocyclic) having a cyclic system of conjugated p orbitals. Representative examples of aromatics include benzene, naphthalene and toluene.

“Carbon-containing compounds” are compounds which comprise carbon. Non-limiting examples of carbon-containing compounds include hydrocarbons, such as methane, ethane and ethylene, CO and CO₂.

“Mixed oxide” or “mixed metal oxide” refers to a catalyst comprising at least two different oxidized metals. In various embodiments, the mixed oxides are “physical blends” of different oxidized metals. For example, in some embodiments, the mixed oxides are represented by M1_(x)O_(z1)/M2_(y)O_(z2), wherein M1 and M2 are the same or different metal elements, O is oxygen and x, y, z1 and z2 are numbers from 1 to 100 to 100 and the “/” indicates that the two oxidized metals are in contact (e.g., physically blended) but not necessarily bound via a covalent or ionic or other type of bond. In other examples, a mixed oxide is a compound comprising two or more oxidized metals and oxygen (e.g., M1_(x)M2_(y)O_(z), wherein M1 and M2 are the same or different metal elements, O is oxygen and x, y and z are numbers from 1 to 100).

A mixed oxide may comprise metal elements in various oxidation states and may comprise more than one type of metal element. For example, a mixed oxide of manganese and magnesium comprises oxidized forms of magnesium and manganese. Each individual manganese and magnesium atom may or may not have the same oxidation state. Mixed oxides comprising 3, 4, 5, 6 or more metal elements can be represented in an analogous manner. Mixed oxides include, but are not limited to metal oxides (M_(x)O_(y)), metal oxyhalides (M_(x)O_(y)X_(z)), metal oxynitrates (M_(x)O_(y)(NO₃)_(z)), metal phosphates (M_(x)(PO₄)_(y)), metal oxycarbonates (M_(x)O_(y)(CO₃)_(z)), metal carbonates, metal oxyhydroxides (M_(x)O_(y)(OH)_(z)) and the like, and combinations thereof, wherein X is independently, at each occurrence, fluoro, chloro, bromo or iodo, and x, y and z are numbers from 1 to 100. Mixed oxides may be represented herein as M1-M2, wherein M1 and M2 are each independently a metal element and M1 and M2 are oxidized. Mixed oxides comprising, 3, 4, 5, 6 or more metal elements can be represented in an analogous manner.

“Rare earth oxide” refers to an oxide of an element from group 3, lanthanides or actinides. Rare earth oxides include mixed oxide containing a rare earth element. Examples of rare earth oxides include, but are not limited to, La₂O₃, Nd₂O₃, Yb₂O₃, Eu₂O₃, Sm₂O₃, Y₂O₃, Ce₂O₃, Pr₂O₃, Ln1_(4-x)Ln2_(x)O₆, La_(4-x)Ln1_(x)O₆, La_(4-x)Nd_(x)O₆, wherein Ln1 and Ln2 are each independently a lanthanide element, wherein Ln1 and Ln2 are not the same and x is a number ranging from greater than 0 to less than 4, La₃NdO₆, LaNd₃O₆, La_(1.5)Nd_(2.5)O₆, La_(2.5)Nd_(1.5)O₆, La_(3.2)Nd_(0.8)O₆, La_(3.5)Nd_(0.5)O₆, La_(3.8)Nd_(0.2)O₆, Y—La, Zr—La, Pr—La and Ce—La.

“O₂-OCM catalyst” refers to a catalyst having activity in the OCM reaction and which predominately uses O₂ as an oxygen source.

“CO₂-OCM catalyst” refers to a catalyst having activity in the OCM reaction and which predominately uses CO₂ as an oxygen source.

“O₂-ODH catalyst” refers to a catalyst having activity in the ODH reaction and which predominately uses O₂ as an oxygen source.

“CO₂-ODH catalyst” refers to a catalyst having activity in the ODH reaction and which predominately uses CO₂ as an oxygen source.

Catalysts

1. Molecular Composition of the Catalysts

As noted above, disclosed herein are catalysts useful in various catalytic reactions. In some embodiments, the catalysts are bulk catalysts (i.e., not nanowire or other nanostructured catalysts). The bulk catalysts can be crystalline or non-crystalline and have various particle sizes.

In other embodiments, the catalysts are nanostructured catalysts, such as nanowires. For example, embodiments of the catalysts are nanowires which are substantially straight such that the ratio of effective length to actual length is 1 or in some embodiments is within about 5%, 4%, 3%, 2% or 1% of 1. In some embodiments, the nanowires are single crystalline.

In other embodiments, the catalysts are nanowires which have a bent morphology. Embodiments of such nanowires can be characterized as having a ratio of effective length to actual length of less than one. In certain embodiments of the bent nanowires, the nanowires have a ratio of effective length to actual length of less than about 0.99, less than about 0.95, less than about 0.90 or even less than about 0.80. In some other embodiments, the nanowires are polycrystalline.

Certain properties of various embodiments of the catalytic nanowires are described in more detail in co-pending U.S. application Ser. No. 13/115,082 (U.S. Pub. No. 2012/0041246), Ser. No. 13/689,514 (U.S. Pub. No. 2013/0158322) and Ser. No. 13/689,611 (U.S. Pub. No. 2013/0165728), which applications were incorporated by reference above. Scanning electron micrographs (SEM) of representative nanowire catalysts are provided in FIGS. 7A and 7B.

In some embodiments, the catalysts comprise one or more metal elements for example, the catalysts may be mono-metallic, bi-metallic, tri-metallic, etc. (i.e., contain one, two, three, etc., metal elements). In some embodiments, the metal elements are present in the catalysts in elemental form while in other embodiments the metal elements are present in oxidized form. In other embodiments the metal elements are present in the catalysts in the form of a compound comprising a metal element. The metal element or compound comprising the metal element may be in the form of oxides (e.g., mixed oxides), hydroxides, carbonates, oxy-hydroxides, oxycarbonates, salts, hydrates, and the like. The metal element or compound comprising the metal element may also be in the form of any of a number of different polymorphs or crystal structures.

Surprisingly, it has been found that addition of dopants to certain metal oxides increases the catalytic activity of the catalyst in the OCM and other reactions.

The present inventors have discovered that erbium, in combination with at least one other lanthanide element, is an effective catalyst for OCM. Accordingly, in one embodiment the heterogeneous catalysts of the present disclosure is a catalyst comprising a mixed oxide base material, the mixed oxide comprising erbium (Er) and at least one further lanthanide element.

In still other embodiments of the foregoing catalyst, the mixed oxide has the following formula (I): Ln_(x)Er_(y)O_(z)  (I) wherein:

Ln is the lanthanide element;

Er is erbium;

O is oxygen; and

x, y and z are each independently numbers greater than 0.

Oxidized lanthanide metals typically exist in the +3 oxidation state, however in the case of Ce, Pr and Tb, the +4 oxidation state is also common and oxides of these metals often include lanthanides in the +3 and/or +4 oxidation states. Accordingly, the values for x, y and z are variable and are limited only by the stable oxidation state of the particular lanthanide and the molar ratio of the lanthanide in the catalyst. For sake of simplicity, some of the specific examples herein (including those examples in Tables 1-20) illustrate lanthanide oxides, wherein the lanthanide metal(s) is in the +3 oxidation state, but it is understood that lanthanide oxides having lanthanides in the +4 and/or a mixture of the +3 and +4 oxidation states are also included within the scope of embodiments of the invention.

Accordingly, in some embodiments x, y and z are selected such that the overall charge of the catalyst is about 0. As used herein, “about 0” with respect to the charge of a catalyst refers to a charge state which is either 0 (i.e., neutral) or close to 0, for example within 5%, 4%, 3%, 2% or 1% of 0. In other embodiments, x, y and z are selected such that z is from 150% to 200% of the sum of x and y. In other embodiments, x, y and z are selected such that z is from 150% to 175% of the sum of x and y. In some other embodiments, x, y and z are selected such that z is from 175% to 200% of the sum of x and y. In different embodiments, x, y and z are selected such that z is about 150% of the sum of x and y. In other different embodiments, x, y and z are selected such that z is about 160% of the sum of x and y. In yet other embodiments, x, y and z are selected such that z is about 170% of the sum of x and y. In more embodiments, x, y and z are selected such that z is about 180% of the sum of x and y. In other embodiments, x, y and z are selected such that z is about 190% of the sum of x and y. In more other embodiments, x, y and z are selected such that z is about 200% of the sum of x and y. In some embodiments, x, y and z are integers (i.e., not fractional numbers), for example integers selected from 1, 2, and 3. In some embodiments, x is 1, y is 1 and z is 3. In other embodiments, x is 3, y is 1, and z is 6. In more embodiments, x is 1, y is 3 and z is 6.

In various embodiments of the foregoing, the mixed oxide comprises LnErO_(3-3.5), Ln₃ErO_(6-7.5) or LnEr₃O_(6-6.5). In various embodiments of the foregoing, the mixed oxide comprises LnErO₃, Ln₃ErO₆ or LnEr₃O₆. In various embodiments of the foregoing, the mixed oxide comprises LnErO₃ or Ln₃ErO₆. In some of these embodiments, Ln is La. In other embodiments, Ln is Ce. In more embodiments, Ln is Pr. In still other embodiments, Ln is Nd. In still more other embodiments, Ln is Sm. In other embodiments, Ln is Eu. In more embodiments, Ln is Gd. In still other embodiments, Ln is Tb. In still more other embodiments, Ln is Dy. In other embodiments, Ln is Ho. In more embodiments, Ln is Tm. In still other embodiments, Ln is Yb. In still more other embodiments, Ln is Lu.

In various other embodiments, the disclosure provides a catalyst comprising a mixed oxide base material, the mixed oxide base material comprising two different lanthanide elements, provided that one of the lanthanide elements is not lanthanum when the other lanthanide element is neodymium. For example, in certain embodiments the disclosure is directed to a catalyst comprising a mixed oxide base material having the following formula (II): Ln1_(a)Ln2_(b)O_(c)  (II) wherein:

Ln1 and Ln2 are independently different lanthanide elements;

O is oxygen; and

a, b and c are each independently numbers greater than 0; and

provided that Ln1 or Ln2 is not neodymium (Nd) when the other of Ln1 or Ln2 is Lanthanum (La).

In some embodiments of the foregoing, Ln1 is Gd and Ln2 is Nd. In some embodiments, the catalyst further comprises a dopant, for example an alkaline earth metal dopant such as calcium. In some embodiments, the catalyst comprises a base material comprising a mixed oxide of Gd and Nd and a calcium dopant.

In other embodiments, a, b and c are selected such that the overall charge of the catalyst is about 0. As used herein, “about 0” with respect to the charge of a catalyst refers to a charge state which is either 0 (i.e., neutral) or close to 0, for example within 5%, 4%, 3%, 2% or 1% of 0. In other embodiments, a, b and c are selected such that c is from 150% to 200% of the sum of a and b. In other embodiments, a, b and c are selected such that c is from 150% to 175% of the sum of a and b. In some other embodiments, a, b and c are selected such that c is from 175% to 200% of the sum of a and b. In different embodiments, a, b and c are selected such that c is about 150% of the sum of a and b. In other different embodiments, a, b and c are selected such that c is about 160% of the sum of a and b. In yet other embodiments, a, b and c are selected such that c is about 170% of the sum of a and b. In more embodiments, a, b and c are selected such that c is about 180% of the sum of a and b. In other embodiments, a, b and c are selected such that c is about 190% of the sum of a and b. In more other embodiments, a, b and c are selected such that c is about 200% of the sum of a and b. In some embodiments, a, b and c are integers, for example integers selected from 1, 2, and 3. In some embodiments, a is 1, b is 1 and c is 3. In other embodiments, a is 3, b is 1, and c is 6. In more embodiments, a is 1, b is 3 and c is 6.

In different embodiments, the disclosure provides a catalyst comprising a mixed oxide base material, the mixed oxide base material comprising three different lanthanide elements. For example, in some embodiments the catalyst comprising a mixed oxide base material has the following formula (III): Ln1_(a)Ln2_(b)Ln3_(d)Ln4_(e)Ln5_(f)O_(c)  (III) wherein:

Ln1, Ln2, Ln3, Ln4 and Ln5 are independently different lanthanide elements;

O is oxygen; and

a, b, c and d are each independently numbers greater than 0; and

e and f are independently 0 or a number greater than 0.

In some embodiments of catalyst (III), a, b, c, d, e and f are selected such that the overall charge of the catalyst is about 0. In other embodiments, a, b, c, d, e and f are selected such that c is from 150% to 200% of the sum of a, b, d, e and f. In other embodiments, a, b, c, d, e and f are selected such that c is from 150% to 175% of the sum of a, b, d, e and f. In some other embodiments, a, b, c, d, e and f are selected such that c is from 175% to 200% of the sum of a, b, d, e and f. In different embodiments, a, b, c, d, e and f are selected such that c is about 150% of the sum of a, b, d, e and f. In other different embodiments, a, b, c, d, e and f are selected such that c is about 160% of the sum of a, b, d, e and f. In yet other embodiments, a, b, c, d, e and f are selected such that c is about 170% of the sum of a, b, d, e and f. In more embodiments, a, b, c, d, e and f are selected such that c is about 180% of the sum of a, b, d, e and f. In other embodiments, a, b, c, d, e and f are selected such that c is about 190% of the sum of a, b, d, e and f. In more other embodiments, a, b, c, d, e and f are selected such that c is about 200% of the sum a, b, d, e and f. In some embodiments, a, b, c, d, e and f are integers, for example integers selected from 1, 2, and 3. In certain embodiments, b is 0. In other embodiments, b is a number greater than 0.

In some embodiments of catalyst (III), a, b, c, d, e and f are selected such that the overall charge of the catalyst is about 0. In certain embodiments, e is 0. In other embodiments, e is a number greater than 0. In certain embodiments, f is 0. In other embodiments, f is a number greater than 0.

With regard to the foregoing catalysts (II and III), the lanthanide elements can be selected from any of the lanthanide elements, provided that Ln1, Ln2, Ln3, Ln4 and Ln5 are different. In some embodiments, Ln1 is La. In other embodiments, Ln1 is Ce. In more embodiments, Ln1 is Pr. In still other embodiments, Ln1 is Nd. In still more other embodiments, Ln1 is Sm. In other embodiments, Ln1 is Eu. In more embodiments, Ln1 is Gd. In still other embodiments, Ln1 is Tb. In still more other embodiments, Ln1 is Dy. In other embodiments, Ln1 is Ho. In other embodiments, Ln1 is Er. In more embodiments, Ln1 is Tm. In still other embodiments, Ln1 is Yb. In still more other embodiments, Ln1 is Lu.

In further embodiments, of the foregoing, Ln1 is selected according to the immediately foregoing paragraph and Ln2 is selected from a different lanthanide. For example, in some embodiments, Ln2 is La. In other embodiments, Ln2 is Ce. In more embodiments, Ln2 is Pr. In still other embodiments, Ln2 is Nd. In still more other embodiments, Ln2 is Sm. In other embodiments, Ln2 is Eu. In more embodiments, Ln2 is Gd. In still other embodiments, Ln2 is Tb. In still more other embodiments, Ln2 is Dy. In other embodiments, Ln2 is Ho. In other embodiments, Ln2 is Er. In more embodiments, Ln2 is Tm. In still other embodiments, Ln2 is Yb. In still more other embodiments, Ln2 is Lu.

In various embodiments, Ln1 is La and Ln2 is Nd. For example, in some embodiments, the catalyst comprises La₃NdO₆.

In other embodiments, Ln1 and Ln2 are selected as above and, Ln3 is La. In other embodiments, Ln3 is Ce. In more embodiments, Ln3 is Pr. In still other embodiments, Ln3 is Nd. In still more other embodiments, Ln3 is Sm. In other embodiments, Ln3 is Eu. In more embodiments, Ln3 is Gd. In still other embodiments, Ln3 is Tb. In still more other embodiments, Ln3 is Dy. In other embodiments, Ln3 is Ho. In other embodiments, Ln3 is Er. In more embodiments, Ln3 is Tm. In still other embodiments, Ln3 is Yb. In still more other embodiments, Ln3 is Lu.

In other embodiments, Ln1, Ln2 and Ln3 are selected as above and, Ln4 is La. In other embodiments, Ln4 is Ce. In more embodiments, Ln4 is Pr. In still other embodiments, Ln4 is Nd. In still more other embodiments, Ln4 is Sm. In other embodiments, Ln4 is Eu. In more embodiments, Ln4 is Gd. In still other embodiments, Ln4 is Tb. In still more other embodiments, Ln4 is Dy. In other embodiments, Ln4 is Ho. In other embodiments, Ln4 is Er. In more embodiments, Ln4 is Tm. In still other embodiments, Ln4 is Yb. In still more other embodiments, Ln4 is Lu.

In other embodiments, Ln1, Ln2, Ln3 and Ln4 are selected as above and, Ln5 is La. In other embodiments, Ln5 is Ce. In more embodiments, Ln5 is Pr. In still other embodiments, Ln5 is Nd. In still more other embodiments, Ln5 is Sm. In other embodiments, Ln5 is Eu. In more embodiments, Ln5 is Gd. In still other embodiments, Ln5 is Tb. In still more other embodiments, Ln5 is Dy. In other embodiments, Ln5 is Ho. In other embodiments, Ln5 is Er. In more embodiments, Ln5 is Tm. In still other embodiments, Ln5 is Yb. In still more other embodiments, Ln5 is Lu.

Further, and in addition to the foregoing proviso with respect to catalyst (II), in some other embodiments Ln1 or Ln2 is not Ce when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Pr when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Pm when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Sm when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Eu when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Gd when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Tb when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Dy when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Ho when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Er when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Tm when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Yb when the other of Ln1 or Ln2 is La. In other embodiments, Ln1 or Ln2 is not Lu when the other of Ln1 or Ln2 is La.

In other embodiments of catalyst (III), none of Ln1, Ln2 or Ln3 is Nd when one other of Ln1, Ln2 or Ln3 is La. In still other embodiments of catalyst (III), none of Ln1, Ln2, Ln3, Ln4 or Ln5 is Nd when one other of Ln1, Ln2, Ln3, Ln4 or Ln5 is La.

In different embodiments of the foregoing catalysts, the mixed oxide comprises various different forms of oxidized metals (e.g., two different lanthanides, such as erbium and one or more other lanthanide). For example, in some embodiments the mixed oxide base material comprises different forms of oxidized metals selected from an oxide, hydroxide, oxy-hydroxide, carbonate, oxy-carbonate, hydroxy carbonate or oxy-hydroxy-carbonate and combinations thereof.

The mixed oxide base material may also be in different forms in various different embodiments, for example in one embodiment the mixed oxide comprises a physical blend of two different lanthanide elements, such as a blend of erbium and another lanthanide.

In various embodiments of any of the foregoing catalysts, the catalyst is a bulk catalyst. In other embodiments, the catalysts are nanostructured catalysts, such as nanowires. For example, embodiments of the catalysts are nanowires which are substantially straight such that the ratio of effective length to actual length is 1 or in some embodiments is within about 5%, 4%, 3%, 2% or 1% of 1. In some embodiments, the nanowires are single crystalline.

In other embodiments, the catalysts are nanowires which have a bent morphology. Embodiments of such nanowires can be characterized as having a ratio of effective length to actual length of less than one. In certain embodiments of the bent nanowires, the nanowires have a ratio of effective length to actual length of less than about 0.99, less than about 0.95, less than about 0.90 or even less than about 0.80. In some other embodiments, the nanowires are polycrystalline.

Although not required for catalytic activity, various embodiments of the forgoing catalysts further comprise one or more doping element. The optional doping elements are selected from any element of the periodic table and are present in concentrations effective to obtain the desired result.

Typically, doping elements are added to a catalyst base material to increase the catalytic activity (e.g., yield, selectivity, conversion, etc.) of the catalyst with respect to a certain reaction, although if desired a doping element which decreases catalytic activity (a poison) may also be added. Accordingly, in one embodiment the catalysts comprise a doping element which is a promoter, for example in one embodiment the doping element is a promoter with respect to the oxidative coupling of methane (i.e., increases the catalytic activity of the base material with respect to the oxidative coupling of methane.

In some embodiments the optional doping element is selected from one or more groups of the periodic table. In some embodiments, the doping element is a lanthanide element. In one embodiment, the catalyst comprises a doping element selected from an element in group 1 of the periodic table. In other embodiments, the catalyst comprises a doping element selected from an element in group 2 of the periodic table. In different embodiments, the catalyst comprises a doping element selected from an element in group 3 of the periodic table. In some other embodiments, the catalyst comprises a doping element selected from an element in group 4 of the periodic table. In still more embodiments, the catalyst comprises a doping element selected from an element in group 5 of the periodic table. In yet other embodiments, the catalyst comprises a doping element selected from an element in group 6 of the periodic table.

In another embodiment, the catalyst comprises a doping element selected from an element in group 7 of the periodic table. In other embodiments, the catalyst comprises a doping element selected from an element in group 8 of the periodic table. In different embodiments, the catalyst comprises a doping element selected from an element in group 9 of the periodic table. In some other embodiments, the catalyst comprises a doping element selected from an element in group 10 of the periodic table. In still more embodiments, the catalyst comprises a doping element selected from an element in group 11 of the periodic table. In yet other embodiments, the catalyst comprises a doping element selected from an element in group 12 of the periodic table.

In still other different embodiments, the catalyst comprises a doping element selected from an element in group 13 of the periodic table. In other embodiments, the catalyst comprises a doping element selected from an element in group 14 of the periodic table. In different embodiments, the catalyst comprises a doping element selected from an element in group 15 of the periodic table. In some other embodiments, the catalyst comprises a doping element selected from an element in group 16 of the periodic table. In still more embodiments, the catalyst comprises a doping element selected from an element in group 17 of the periodic table.

Combinations of different doping elements from different groups of the periodic table are also included in various other embodiments. For example, in some embodiments the catalysts comprise two or more doping elements selected from groups 2 and 4, groups 6 and 13, groups 4 and 13, groups 2 and 6, groups 2 and 13 or groups 4 and 6. In other embodiments, the catalysts comprise one or more doping element selected from groups 1, 2, 3, 4, 6 and 13 of the periodic table. In still other embodiments, the catalysts comprise one or more doping element selected from groups 2, 4, 6 and 13 of the periodic table.

In still other embodiments, the catalyst comprises at least one dopant from any of groups 1-17 and second dopant selected from any one of groups 1-17. In certain embodiments, the second doping element is selected from groups 1, 2, 3, 4, 6, 13 and the lanthanides. In some embodiments, the second doping element is selected from an element in group 1 of the periodic table. In other embodiments, the second doping element is selected from an element in group 2 of the periodic table. In more embodiments, the second doping element is selected from an element in group 3 of the periodic table. In still other embodiments, the second doping element is selected from an element in group 4 of the periodic table. In yet more embodiments, the second doping element is selected from an element in group 6 of the periodic table. In still other embodiments, the second doping element is selected from an element in group 13 of the periodic table. In more other embodiments, the second doping element is selected from a lanthanide element.

In still other embodiments, the catalyst comprises at least two doping elements, wherein the first doping element is selected as described above and the second doping element is selected from Ce, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, B, Ba, Sr, Mg, Ca, K, W, Hf, Ga, Y and Al. In some embodiments, the second doping element is Ce. In other embodiments, the second doping element is Sm. In more embodiments, the second doping element is Eu. In yet more embodiments, the second doping element is Gd. In some embodiments, the second doping element is Tb. In other embodiments, the second doping element is Dy. In more embodiments, the second doping element is Ho. In yet more embodiments, the second doping element is Er. In some embodiments, the second doping element is Tm. In other embodiments, the second doping element is Yb. In more embodiments, the second doping element is Lu. In yet more embodiments, the second doping element is B. In some embodiments, the second doping element is Ba. In other embodiments, the second doping element is Sr. In more embodiments, the second doping element is Mg. In yet more embodiments, the second doping element is Ca. In some embodiments, the second doping element is K. In other embodiments, the second doping element is W. In more embodiments, the second doping element is Hf. In yet more embodiments, the second doping element is Ga. In some embodiments, the second doping element is Y. In other embodiments, the second doping element is Al.

In some embodiments, the doping element is selected from Mg, Ca, Sr and Ba. In other embodiments, the doping element is selected from Sr and Ba. In yet other embodiments, the doping element is Zr or Hf. In various embodiments of the foregoing, the catalyst comprises a doping element selected from Mg, Ca, Sr, Ba, Zr and Hf and a second doping element selected from groups 1, 2 and the lanthanides. For example in some embodiments, the second doping is Sr, K, Ba, Na, Eu or Tm. In some embodiments the second doping element is Sr. In other embodiments, the second doping element is K. In some embodiments the second doping element is Ba. In other embodiments, the second doping element is Na. In some embodiments the second doping element is Eu. In other embodiments, the second doping element is Tm.

In some embodiments, the doping element is selected from an element in group 6, such as W. In various further embodiments, such catalysts further comprise a doping element selected from the lanthanides, such as Er. In this regard the Er is considered a doping element and is present in addition to the Er already present in the catalyst.

In still other embodiments, the catalysts comprise a doping element selected from group 13 of the periodic table, for example B, Al or Ga. In some embodiments, the doping element is B. In some other embodiments, the doping element is Al. In some more embodiments, the doping element is Ga. In further embodiments of the foregoing, the catalyst further comprises a doping element from group 2 of the periodic table, for example Sr, Ba, Ca or Mg. In some embodiments, this further doping element is Sr. In other embodiments, the further doping element is Ba. In other embodiments, the further doping element is Ca. In other embodiments, the further doping element is Mg.

In some embodiments, the doping element is selected from one or more elements in groups 2, 6 and 13. For example, in some embodiments the element is selected from Sr, Ba, W and B. In some embodiments, the element is Sr. In some embodiments, the element is Ba. In some embodiments, the element is W. In some embodiments, the element is B.

Combinations of Sr, Ba, W and B as doping elements are also within the scope of certain embodiments of the invention. For example, in addition to the base catalyst, some embodiments comprise doping combinations selected from Sr/Ba, Sr/W and Sr/B. In other embodiments, the doping combination is selected from Ba/Sr, Ba/W and Ba/B. In other embodiments, the dopants comprise W/Sr. W/Ba or W/B. In still other embodiments, the dopants comprise B/Sr, B/Ba or B/W.

In still more embodiments, the doping elements comprise one of the following combinations: Sr/Ba/W, Sr/Ba/B, Sr/W/B or Ba/W/B. In some embodiments, the dopant combination comprises Sr/Ba/W/B.

In other embodiments, the catalyst comprises one of the following doping combinations: W, Sr, Ba, B, Sr/Ce, Sr/Sm, Sr/Eu, Sr/Gd, Sr/Tb, Sr/Dy, Sr/Er, Sr/Tm, Sr/Yb, Sr/Lu, Sr/Ba/B, Sr/B, Ba/B, Ba/Sr, Er/W, Sr/K, Ba/Ce, Ca/W, Sr/Hf, Sr/Hf/K, Ba/Hf, Ga/Mg, Sr/Ca, Y/Ba, Sr/Ga/Mg, Sr/Y, Na/Zr/Eu/Tm, Sr/B/Y, Ca/B, Sr/Ga, Sr/Al, Sr/W, Ba/W, B/W, Sr/Ba/W, Sr/W/B, Ba/W/B or Sr/Ba/W/B.

The doping elements, when present, in the foregoing mixed oxides is selected from any of groups 1-17, and in some embodiments the doping element is selected according to any of the foregoing described embodiments.

In various other embodiments, the disclosure provides a catalyst comprising a base material comprising an oxide of one or more lanthanide elements and a dopant combination selected from Sr/Sm, Sr/Gd, Sr/Dy, Sr/Er, Sr/Lu, Sr/Ba/B, Ba/B, Ba/Sr, Er/W, Sr/K, Ba/Ce, Ba/Hf, Ga/Mg, Mg/Er, Y/Ba, Sr/Ga/Mg, Sr/Y, Sr/B/Y, Ca/B, Sr/Al, Ba/W, B/W, Sr/Ba/W, Sr/W/B, Ba/W/B and Sr/Ba/W/B.

In various embodiments, the foregoing oxide has the following formula (III): Ln1_(a)Ln2_(b)Ln3_(d)Ln4_(e)Ln5_(f)O_(c)  (III) wherein:

Ln1, Ln2, Ln3, Ln4 and Ln5 are independently different lanthanide elements;

O is oxygen; and

a and c are each independently numbers greater than 0; and

b, d, e and f are independently 0 or a number greater than 0.

In various embodiments of the foregoing catalyst (III), Ln1, Ln2, Ln3, Ln4 and Ln5 are selected according to the foregoing description with respect to catalyst (III). Further embodiments, include embodiments wherein a, b, c, d, e and f are selected according to the foregoing description with respect to catalyst (III).

In some embodiments, the dopant combination consists essentially of Sr/Sm, Sr/Gd, Sr/Dy, Sr/Er, Sr/Lu, Sr/Ba/B, Ba/B, Ba/Sr, Er/W, Sr/K, Ba/Ce, Ba/Hf, Ga/Mg, Mg/Er, Y/Ba, Sr/Ga/Mg, Sr/Y, Sr/B/Y, Ca/B, Sr/Al, Ba/W, B/W, Sr/Ba/W, Sr/W/B, Ba/W/B or Sr/Ba/W/B. In other embodiments, the dopant combination consists of Sr/Sm, Sr/Gd, Sr/Dy, Sr/Er, Sr/Lu, Sr/Ba/B, Ba/B, Ba/Sr, Er/W, Sr/K, Ba/Ce, Ba/Hf, Ga/Mg, Mg/Er, Y/Ba, Sr/Ga/Mg, Sr/Y, Sr/B/Y, Ca/B, Sr/Al, Ba/W, B/W, Sr/Ba/W, Sr/W/B, Ba/W/B or Sr/Ba/W/B. In some embodiments, the foregoing catalyst (III) is a bulk catalyst and in other embodiments the foregoing catalyst (III) is a nanowire catalyst.

In other embodiments, the present disclosure provides a bulk catalyst comprising a base material comprising an oxide of one or more lanthanide elements and a dopant combination selected from Sr/Ce, Sr/Tb, Sr/B and Sr/Hf/K.

In some embodiments, the bulk catalyst oxide has the following formula (III): Ln1_(a)Ln2_(b)Ln3_(d)Ln4_(e)Ln5_(f)O_(c)  (III) wherein:

Ln1, Ln2, Ln3, Ln4 and Ln5 are independently different lanthanide elements;

O is oxygen; and

a and c are each independently numbers greater than 0; and

b, d, e, and f are independently 0 or a number greater than 0.

In various embodiments of the foregoing bulk catalyst (III), Ln1, Ln2, Ln3, Ln4 and Ln5 are selected according to the foregoing description with respect to catalyst (III). Further embodiments, include embodiments wherein a, b, c, d, e and f are selected according to the foregoing description with respect to catalyst (III).

In other the dopant combination consists essentially of Sr/Ce, Sr/Tb, Sr/B or Sr/Hf/K. In still more embodiments, the dopant combination consists of Sr/Ce, Sr/Tb, Sr/B or Sr/Hf/K.

It should be noted that the foregoing catalyst formulas (I, II and III) refer to the base catalyst formula and do not include dopants. For example, the lanthanide elements (i.e., Ln, Ln1, Ln2, Ln3, Ln4 or Ln5) illustrated as part of the foregoing catalysts are part of the base catalyst and not a dopant. Instead, dopants are an optional feature in addition to the base catalyst material. Dopants are generally, but not always, present in lower quantities than the elements in the base material.

In other embodiments is provided a catalyst comprising a mixture of a Group 3 element and a Group 4 element or lanthanide element, the catalyst further comprising an alkaline earth metal dopant. In some of the embodiments, the catalyst comprises one of the following mixtures of elements: Y/Zr, Y/Ti, Y/Gd, Y/Ce, Y/La, Y/Ca, Y/Ti, Y/Eu. In various embodiments, the alkaline earth metal dopant is barium, strontium or calcium.

In some embodiments, the catalyst comprises Ca/Sr/Y/Zr. In some other embodiments, the catalyst comprises Nd/Ca/Sr/Y/Zr. In some different embodiments, the catalyst comprises Ca/Y/Ti. In yet other embodiments, the catalyst comprises Ba/Gd/Y. In more embodiments, the catalyst comprises Ba/Ce/Y. In some other embodiments, the catalyst comprises Ba/La/Y. In other embodiments, the catalyst comprises Ba/Ca/Y. In some embodiments, the catalyst comprises Ba/Y/Ti. In yet other different embodiments, the catalyst comprises Ba/Eu/Y.

In still other embodiments, a catalyst comprising a Group 4 or lanthanide oxide in combination with an alkaline earth metal dopant is provided. In some of these embodiments, the dopant is barium or strontium. In some specific embodiments, the catalyst comprises Ba/HfO₃. In some other embodiments, the catalyst comprises Sr/HfO₃. In more embodiments, the catalyst comprises Ba/CeO₃. In still other embodiments, the catalyst comprises Ba/TiO₃. In some other embodiments, the catalyst comprises Ba/W/Nd₂O₃. In some different embodiments, the catalyst comprises Ba/W/Er₂O₃.

In certain embodiments of any of the foregoing catalysts, the catalysts comprises a C₂+ selectivity of greater than 50% and a methane conversion of greater than 20% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

In other embodiments, the C₂+ selectivity of the catalysts is greater than 60% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

In other embodiments, the catalysts have a C2+ yield greater than 10%, or even greater than 15% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

In some other embodiments, the catalysts comprise a methane conversion greater than 20% or greater than 30% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

In other certain embodiments, the catalysts are capable of methane conversions in an OCM reaction of greater than 20% and C₂+ selectivities of greater than 50% at temperatures ranging from about 550 C to about 750 C, for example, from about 600 C to about 700 C. In other embodiments of the foregoing, the methane conversion is greater than 22%, greater than 24% or even greater than 26%. In still other embodiments of any of the foregoing, the C2+ selectivity of the catalysts is greater than 55% or even greater than 60%.

In various embodiments, of any of the above catalysts, the catalyst comprises a C2+ selectivity of greater than 50% and a methane conversion of greater than 20% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less, 700° C. or less, 650° C. or less or even 600° C. or less.

In more embodiments, of any of the above catalysts, the catalyst comprises a C2+ selectivity of greater than 50%, greater than 55%, greater than 60%, greater than 65%, greater than 70%, or even greater than 75%, and a methane conversion of greater than 20% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.

In other embodiments, of any of the above catalysts, the catalyst comprises a C2+ selectivity of greater than 50%, and a methane conversion of greater than 20%, greater than 25%, greater than 30%, greater than 35%, greater than 40%, greater than 45%, or even greater than 50% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less. In some embodiments of the foregoing, the methane conversion and C2+ selectivity are calculated based on a single pass basis (i.e., the percent of methane converted or C2+ selectivity upon a single pass over the catalyst or catalytic bed, etc.)

In various embodiments, the foregoing performance parameters of the catalysts (e.g., conversion, yield, selectivity) are determined when the oxidative coupling of methane is performed at a temperature of 700° C. or less. In other embodiments, the parameters are determined when the oxidative coupling of methane is performed at reaction inlet pressures ranging from 1 atm to 16 atm.

In various embodiments of the foregoing, the methane conversion, C₂₊ selectivity, or C2+ yield, or combinations thereof, are measured in a 4 millimeter inner diameter tube with a methane to oxygen ratio of 5.5:1 using air as an oxidant, wherein the temperature is 650° C.

In various other embodiments, the oxidative coupling of methane is performed at a temperature of 700° C. or less, and in some different embodiments, the oxidative coupling of methane is performed at reaction inlet pressures ranging from 1 atm to 16 atm.

The metal oxides disclosed herein can be in the form of oxides, oxyhydroxides, hydroxides, oxycarbonates or combination thereof after being exposed to moisture, carbon dioxide, undergoing incomplete calcination or combination thereof.

The foregoing catalysts comprise 0, 1, 2, 3, 4 or more doping elements. In this regard, each dopant may be present in the catalysts (for example any of the catalysts described above and/or disclosed in Tables 1-20) in up to 75% by weight of the catalyst. For example, in one embodiment the concentration of a first doping element (when present) ranges from 0.01% to 1% w/w, 1%-5% w/w, 5%-10% w/w, 10%-20% w/w, 20%-30% w/w, 30%-40% w/w or 40%-50% w/w, for example about 1% w/w, about 2% w/w, about 3% w/w, about 4% w/w, about 5% w/w, about 6% w/w, about 7% w/w, about 8% w/w, about 9% w/w, about 10% w/w, about 11% w/w, about 12% w/w, about 13% w/w, about 14% w/w, about 15% w/w, about 16% w/w, about 17% w/w, about 18% w/w, about 19% w/w or about 20% w/w.

In other embodiments, the concentration of a second doping element (when present) ranges from 0.01% to 1% w/w, 1%-5% w/w, 5%-10% w/w, 10%-20% w/w, 20%-30% w/w, 30%-40% w/w or 40%-50% w/w, for example about 1% w/w, about 2% w/w, about 3% w/w, about 4% w/w, about 5% w/w, about 6% w/w, about 7% w/w, about 8% w/w, about 9% w/w, about 10% w/w, about 11% w/w, about 12% w/w, about 13% w/w, about 14% w/w, about 15% w/w, about 16% w/w, about 17% w/w, about 18% w/w, about 19% w/w or about 20% w/w.

In other embodiments, the concentration of a third doping element (when present) ranges from 0.01% to 1% w/w, 1%-5% w/w, 5%-10% w/w, 10%-20% w/w, 20%-30% w/w, 30%-40% w/w or 40%-50% w/w, for example about 1% w/w, about 2% w/w, about 3% w/w, about 4% w/w, about 5% w/w, about 6% w/w, about 7% w/w, about 8% w/w, about 9% w/w, about 10% w/w, about 11% w/w, about 12% w/w, about 13% w/w, about 14% w/w, about 15% w/w, about 16% w/w, about 17% w/w, about 18% w/w, about 19% w/w or about 20% w/w.

In other embodiments, the concentration of a fourth doping element (when present) ranges from 0.01% to 1% w/w, 1%-5% w/w, 5%-10% w/w, 10%-20% w/w, 20%-30% w/w, 30%-40% w/w or 40%-50% w/w, for example about 1% w/w, about 2% w/w, about 3% w/w, about 4% w/w, about 5% w/w, about 6% w/w, about 7% w/w, about 8% w/w, about 9% w/w, about 10% w/w, about 11% w/w, about 12% w/w, about 13% w/w, about 14% w/w, about 15% w/w, about 16% w/w, about 17% w/w, about 18% w/w, about 19% w/w or about 20% w/w.

In other embodiments, the concentration of the dopant is measured in terms of atomic percent (at/at). In some of these embodiments, each dopant may be present in the catalysts (for example any of the catalysts described above and/or disclosed in Tables 1-20) in up to 75% at/at. For example, in one embodiment the concentration of a first doping element (when present) ranges from 0.01% to 1% at/at, 1%-5% at/at, 5%-10% at/at, 10%-20% at/at, 20%-30% at/at, 30%-40% at/at or 40%-50% at/at, for example about 1% at/at, about 2% at/at, about 3% at/at, about 4% at/at, about 5% at/at, about 6% at/at, about 7% at/at, about 8% at/at, about 9% at/at, about 10% at/at, about 11% at/at, about 12% at/at, about 13% at/at, about 14% at/at, about 15% at/at, about 16% at/at, about 17% at/at, about 18% at/at, about 19% at/at or about 20% at/at.

In other embodiments, the concentration of a second doping element (when present) ranges from 0.01% to 1% at/at, 1%-5% at/at, 5%-10% at/at, 10%-20% w/w, 20%-30% at/at, 30%-40% at/at or 40%-50% at/at, for example about 1% at/at, about 2% at/at, about 3% at/at, about 4% at/at, about 5% at/at, about 6% at/at, about 7% at/at, about 8% at/at, about 9% at/at, about 10% at/at, about 11% at/at, about 12% at/at, about 13% at/at, about 14% at/at, about 15% at/at, about 16% at/at, about 17% at/at, about 18% at/at, about 19% at/at or about 20% at/at.

In other embodiments, the concentration of a third doping element (when present) ranges from 0.01% to 1% at/at, 1%-5% at/at, 5%-10% at/at, 10%-20% w/w, 20%-30% at/at, 30%-40% at/at or 40%-50% at/at, for example about 1% at/at, about 2% at/at, about 3% at/at, about 4% at/at, about 5% at/at, about 6% at/at, about 7% at/at, about 8% at/at, about 9% at/at, about 10% at/at, about 11% at/at, about 12% at/at, about 13% at/at, about 14% at/at, about 15% at/at, about 16% at/at, about 17% at/at, about 18% at/at, about 19% at/at or about 20% at/at.

In other embodiments, the concentration of a fourth doping element (when present) ranges from 0.01% to 1% at/at, 1%-5% at/at, 5%-10% at/at, 10%-20% w/w, 20%-30% at/at, 30%-40% at/at or 40%-50% at/at, for example about 1% at/at, about 2% at/at, about 3% at/at, about 4% at/at, about 5% at/at, about 6% at/at, about 7% at/at, about 8% at/at, about 9% at/at, about 10% at/at, about 11% at/at, about 12% at/at, about 13% at/at, about 14% at/at, about 15% at/at, about 16% at/at, about 17% at/at, about 18% at/at, about 19% at/at or about 20% at/at.

Accordingly, any of the doped catalysts described above or in Tables 1-20, may comprise any of the foregoing doping concentrations.

Furthermore, different catalytic characteristics of the above doped catalysts can be varied or “tuned” based on the method used to prepare them. Such methods are described in more detail herein and other methods are known in the art. In addition, the above dopants may be incorporated either before or after (or combinations thereof) an optional calcination step as described herein.

Tables 1-20 below show exemplary doped catalysts in accordance with various specific embodiments. Dopants are shown in the horizontal rows and base catalyst in the vertical columns in tables 1-8. Dopants are shown in the vertical columns and base catalyst in the horizontal rows in table 9-20. The resulting doped catalysts are shown in the intersecting cells. The variables a, b, c, x, y and z in Tables 1-20 are as defined above herein.

Lengthy table referenced here US09956544-20180501-T00001 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00002 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00003 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00004 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00005 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00006 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00007 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00008 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00009 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00010 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00011 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00012 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00013 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00014 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00015 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00016 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00017 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00018 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00019 Please refer to the end of the specification for access instructions.

Lengthy table referenced here US09956544-20180501-T00020 Please refer to the end of the specification for access instructions.

The catalysts of the disclosure may be analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine the element content of the catalysts. ICP-MS is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 10¹². ICP is based on coupling together an inductively coupled plasma as a method of producing ions (ionization) with a mass spectrometer as a method of separating and detecting the ions. ICP-MS methods are well known in the art.

As used throughout the specification, a catalyst composition represented by E¹/E²/E³, etc., wherein E¹, E² and E³ are each independently an element or a compound comprising one or more elements, refers to a catalyst comprised of a mixture of E¹, E² and E³. E¹, E² and E³, etc., are not necessarily present in equal amounts and need not form a bond with one another. For example, a catalyst comprising Li/MgO refers to a catalyst comprising Li and MgO, for example, Li/MgO may refer to MgO doped with Li. By way of another example, a catalyst comprising Na/Mn/W/O refers to a catalyst comprised of a mixture of sodium, manganese, tungsten and oxygen. Generally the oxygen is in the form of a metal oxide.

In some embodiments, dopants are present in the catalysts in, for example, less than 50 at %, less than 25 at %, less than 10 at %, less than 5 at % or less than 1 at %.

In other embodiments of the catalysts, the weight ratio (w/w) of the catalyst base material to the doping element(s) ranges from 1:1 to 10,000:1, 1:1 to 1,000:1 or 1:1 to 500:1.

2. Catalytic Materials

The present disclosure includes a catalytic material comprising a plurality of catalysts. In certain embodiments, the catalytic material comprises a support or carrier. Supports and carriers useful in the context of the invention are not limited and include supports and carriers described herein as well as those known in the art, for example as described in U.S. application Ser. No. 13/115,082 (U.S. Pub. No. 2012/0041246), Ser. No. 13/479,767 (U.S. Pat. No. 8,921,256), Ser. No. 13/689,514 (U.S. Pub. No. 2013/0158322), Ser. No. 13/689,611 (U.S. Pub. No. 2013/0165728) and Ser. No. 13/901,319 (corresponding to PCT Pub. No. WO 2013/177461), and PCT App. No. US 2014/028040, the full disclosures of which are hereby incorporated by reference in their entireties.

The support is preferably porous and has a high surface area. In some embodiments the support is active (i.e., has catalytic activity). In other embodiments, the support is inactive (i.e., non-catalytic). In some embodiments, the support comprises an inorganic oxide, Al₂O₃, SiO₂, TiO₂, MgO, CaO, SrO, ZrO₂, ZnO, LiAlO₂, MgAl₂O₄, MnO, MnO₂, Mn₃O₄, La₂O₃, AlPO₄, SiO₂/Al₂O₃, activated carbon, silica gel, zeolites, activated clays, activated Al₂O₃, SiC, diatomaceous earth, magnesia, aluminosilicates, calcium aluminate, support nanowires or combinations thereof. In some embodiments the support comprises silicon, for example SiO₂. In other embodiments the support comprises magnesium, for example MgO. In yet other embodiments, the support comprises yttrium, for example Y₂O₃. In other embodiments the support comprises zirconium, for example ZrO₂. In yet other embodiments, the support comprises lanthanum, for example La₂O₃. In yet other embodiments, the support comprises lanthanum, for example La₂O₃. In yet other embodiments, the support comprises hafnium, for example HfO₂. In yet other embodiments, the support comprises aluminum, for example Al₂O₃. In yet other embodiments, the support comprises gallium, for example Ga₂O₃.

In still other embodiments, the support material comprises an inorganic oxide, Al₂O₃, SiO₂, TiO₂, MgO, ZrO₂, HfO₂, CaO, SrO, ZnO, LiAlO₂, MgAl₂O₄, MnO, MnO₂, Mn₂O₄, Mn₃O₄, La₂O₃, AlPO₄, activated carbon, silica gel, zeolites, activated clays, activated Al₂O₃, diatomaceous earth, magnesia, aluminosilicates, calcium aluminate, support nanowires or combinations thereof. In yet other embodiments, a catalyst may serve as a support for another catalyst. For example, a catalyst may be comprised of catalytic support material and adhered to or incorporated within the support is another catalyst. For example, in some embodiments, the catalytic support may comprise SiO₂, MgO, TiO₂, ZrO₂, Al₂O₃, ZnO or combinations thereof.

In still other embodiments, the support material comprises a carbonate. For example, in some embodiments the support material comprises MgCO₃, CaCO₃, SrCO₃, BaCO₃, Y₂(CO₃)₃, La₂(CO₃)₃ or combinations thereof.

In other embodiments, the support material is selected from AlPO₄, Al₂O₃, SiO₂—Al₂O₃, CaO, TiO₂, ZrO₂, MgO, SiO₂, HfO₂, In₂O₃, SiC and combinations thereof.

In yet other embodiments, a nanowire may serve as a support for another bulk or nanowire catalyst. For example, a nanowire may be comprised of non-catalytic metal elements and adhered to or incorporated within the support nanowire is a catalyst as described herein. For example, in some embodiments, the support nanowires are comprised of SiO₂, MgO, CaO, SrO, TiO₂, ZrO₂, Al₂O₃, ZnO, MgCO₃, CaCO₃, SrCO₃ or combinations thereof. The optimum amount of catalyst present on the support depends, inter alia, on the catalytic activity of the catalyst. In some embodiments, the amount of catalyst present on the support ranges from 1 to 100 parts by weight of catalyst per 100 parts by weight of support or from 10 to 50 parts by weight of catalyst per 100 parts by weight of support. In other embodiments, the amount of catalyst present on the support ranges from 100-200 parts of catalyst per 100 parts by weight of support, or 200-500 parts of catalyst per 100 parts by weight of support, or 500-1000 parts of catalyst per 100 parts by weight of support. Typically, heterogeneous catalysts are used either in their pure form or blended with inert materials, such as silica, alumina, etc. The blending with inert materials is used in order to reduce and/or control large temperature non-uniformities within the reactor bed often observed in the case of strongly exothermic (or endothermic) reactions. In the case of complex multistep reactions, such as the reaction to convert methane into ethylene (OCM), typical blending materials can selectively slow down or quench one or more of the reactions of the system and promote unwanted side reactions. For example, in the case of the oxidative coupling of methane, silica and alumina can quench the methyl radicals and thus prevent the formation of ethane. In certain aspects, the present disclosure provides a catalytic material which solves these problems typically associated with catalyst support material. Accordingly, in certain embodiments the catalytic activity of the catalytic material can be tuned by blending two or more catalysts and/or catalyst support materials. The blended catalytic material may comprise a catalyst as described herein in combination with another catalytic material, for example an additional bulk catalyst or a catalytic nanowire as described in PCT Pub. Nos. WO 2011/14996; WO 2013/082318 and WO 2012/162526 which are hereby incorporated by reference in their entireties, and/or inert support material.

In some embodiments, the blended catalytic materials comprise metal oxides, hydroxides, oxy-hydroxides, carbonates, oxalates of the groups 1-16, lanthanides, actinides or combinations thereof. For example, the blended catalytic materials may comprise a plurality of catalysts, as disclosed herein, and any one or more of straight nanowires, nanoparticles, bulk materials and inert support materials. The blended catalytic materials may be undoped or may be doped with any of the dopants described herein or other dopants useful for obtaining the desired catalytic activity.

In one embodiment, the catalyst blend comprises at least one type 1 component and at least one type 2 component. Type 1 components comprise catalysts having a high OCM activity at moderately low temperatures and type 2 components comprise catalysts having limited or no OCM activity at these moderately low temperatures, but are OCM active at higher temperatures. For example, in some embodiments the type 1 component is a catalyst having high OCM activity at moderately low temperatures. For example, the type 1 component may comprise a C2+ yield of greater than 5% or greater than 10% at temperatures less than 800° C., less than 700° C. or less than 600° C. The type 2 component may comprise a C2+ yield less than 0.1%, less than 1% or less than 5% at temperatures less than 800° C., less than 700° C. or less than 600° C. The type 2 component may comprise a C2+ yield of greater than 0.1%, greater than 1%, greater than 5% or greater than 10% at temperatures greater than 800° C., greater than 700° C. or greater than 600° C. Typical type 1 components include any of the catalysts as described herein, while typical type 2 components include bulk OCM catalysts and nanowire catalysts which only have good OCM activity at higher temperatures, for example greater than 800° C. Examples of type 2 components may include catalysts comprising MgO. The catalyst blend may further comprise inert support materials as described above (e.g., silica, alumina, silicon carbide, etc.).

In certain embodiments, the type 2 component acts as diluent in the same way an inert material does and thus helps reduce and/or control hot spots in the catalyst bed caused by the exothermic nature of the OCM reaction. However, because the type 2 component is an OCM catalyst, albeit not a particularly active one, it may prevent the occurrence of undesired side reactions, e.g., methyl radical quenching. Additionally, controlling the hotspots has the beneficial effect of extending the lifetime of the catalyst.

For example, it has been found that diluting active lanthanide oxide OCM catalysts with as much as a 10:1 ratio of MgO, which by itself is not an active OCM catalyst at the temperature which the lanthanide oxide operates, is a good way to minimize “hot spots” in the reactor catalyst bed, while maintaining the selectivity and yield performance of the catalyst. On the other hand, doing the same dilution with quartz SiO₂ is not effective because it appears to quench the methyl radicals which serves to lower the selectivity to C2s.

In yet another embodiment, the type 2 components are good oxidative dehydrogenation (ODH) catalysts at the same temperature that the type 1 components are good OCM catalysts. In this embodiment, the ethylene/ethane ratio of the resulting gas mixture can be tuned in favor of higher ethylene. In another embodiment, the type 2 components are not only good ODH catalysts at the same temperature the type 1 components are good OCM catalysts, but also have limited to moderate OCM activity at these temperatures.

In related embodiments, the catalytic performance of the catalytic material is tuned by selecting specific type 1 and type 2 components of a catalyst blend. In another embodiment, the catalytic performance is tuned by adjusting the ratio of the type 1 and type 2 components in the catalytic material. For example, the type 1 catalyst may be a catalyst for a specific step in the catalytic reaction, while the type 2 catalyst may be specific for a different step in the catalytic reaction. For example, the type 1 catalyst may be optimized for formation of methyl radicals and the type 2 catalyst may be optimized for formation of ethane or ethylene.

In other embodiments, the catalytic material comprises at least two different components (component 1, component 2, component 3, etc.). The different components may comprise different morphologies, e.g., nanowires, nanoparticles, bulk, etc. The different components in the catalyst material can be, but not necessarily, of the same chemical composition and the only difference is in the morphology and/or the size of the particles. This difference in morphology and particle size may result in a difference in reactivity at a specific temperature. Additionally, the difference in morphology and particle size of the catalytic material components is advantageous for creating a very intimate blending, e.g., very dense packing of the catalysts particles, which can have a beneficial effect on catalyst performance. Also, the difference in morphology and particle size of the blend components would allow for control and tuning of the macro-pore distribution in the reactor bed and thus its catalytic efficiency. An additional level of micro-pore tuning can be attained by blending catalysts with different chemical composition and different morphology and/or particle size. The proximity effect would be advantageous for the reaction selectivity.

Accordingly, in one embodiment the present disclosure provides the use of a catalytic material comprising a first catalyst and a second catalyst, for example a first catalytic nanowire and a bulk catalyst and/or a second catalytic nanowire, in a catalytic reaction, for example the catalytic reaction may be OCM or ODH. In other embodiments, the first catalytic nanowire and the bulk catalyst and/or second catalytic nanowire are each catalytic with respect to the same reaction, and in other examples the first catalytic nanowire and the bulk catalyst and/or second catalytic nanowire have the same chemical composition.

In some specific embodiments of the foregoing, the catalytic material comprises a first catalytic nanowire and a second catalytic nanowire. Each nanowire can have completely different chemical compositions or they may have the same base composition and differ only by the doping elements. In other embodiments, each nanowire can have the same or a different morphology. For example, each nanowire can differ by the nanowire size (length and/or aspect ratio), by ratio of actual/effective length, by chemical composition or any combination thereof. Furthermore, the first and second nanowires may each be catalytic with respect to the same reaction but may have different activity. Alternatively, each nanowire may catalyze different reactions.

In a related embodiment, the catalytic material comprises a first catalytic nanowire and a bulk catalyst. The first nanowire and the bulk catalyst can have completely different chemical compositions or they may have the same base composition and differ only by the doping elements. Furthermore, the first nanowire and the bulk catalyst may each be catalytic with respect to the same reaction but may have different activity. Alternatively, the first nanowire and the bulk catalyst may catalyze different reactions.

In yet other embodiments of the foregoing, the catalytic nanowire has a catalytic activity in the catalytic reaction, which is greater than a catalytic activity of the bulk catalyst in the catalytic reaction at the same temperature. In still other embodiments, the catalytic activity of the bulk catalyst in the catalytic reaction increases with increasing temperature.

OCM catalysts may be prone to hotspots due to the very exothermic nature of the OCM reaction. Diluting such catalysts helps to manage the hotspots. However, the diluent needs to be carefully chosen so that the overall performance of the catalyst is not degraded. Silicon carbide for example can be used as a diluent with little impact on the OCM selectivity of the blended catalytic material whereas using silica as a diluent significantly reduces OCM selectivity. The good heat conductivity of SiC is also beneficial in minimizing hot spots. As noted above, use of a catalyst diluents or support material that is itself OCM active has significant advantages over more traditional diluents such as silica and alumina, which can quench methyl radicals and thus reduce the OCM performance of the catalyst. An OCM active diluent is not expected to have any adverse impact on the generation and lifetime of methyl radicals and thus the dilution should not have any adverse impact on the catalyst performance. Thus embodiments of the invention include catalyst compositions comprising an OCM catalyst (e.g., any of the disclosed catalysts) in combination with a diluent or support material that is also OCM active. Methods for use of the same in an OCM reaction are also provided.

In some embodiments, the above diluent comprises alkaline earth metal compounds, for example alkaline metal oxides, carbonates, sulfates or phosphates. Examples of diluents useful in various embodiments include, but are not limited to, MgO, MgCO₃, MgSO₄, Mg₃(PO₄)₂, MgAl₂O₄, CaO, CaCO₃, CaSO₄, Ca₃(PO₄)₂, CaAl₂O₄, SrO, SrCO₃, SrSO₄, Sr₃(PO₄)₂, SrAl₂O₄, BaO, BaCO₃, BaSO₄, Ba₃(PO₄)₂, BaAl₂O₄ and the like. Most of these compounds are very cheap, especially MgO, CaO, MgCO₃, CaCO₃, SrO, SrCO₃ and thus very attractive for use as diluents from an economic point of view. Additionally, the magnesium, calcium and strontium compounds are environmentally friendly too. Accordingly, an embodiment of the invention provides a catalytic material comprising a catalyst in combination with a diluent selected from one or more of MgO, MgCO₃, MgSO₄, Mg₃(PO₄)₂, CaO, CaCO₃, CaSO₄, Ca₃(PO₄)₂, SrO, SrCO₃, SrSO₄, Sr₃(PO₄)₂, BaO, BaCO₃, BaSO₄, Ba₃(PO₄)₂. In some specific embodiments the diluents is MgO, CaO, SrO, MgCO₃, CaCO₃, SrCO₃ or combination thereof. Methods for use of the foregoing catalytic materials in an OCM reaction are also provided. The methods comprise converting methane to ethane and or ethylene in the presence of the catalytic materials.

The above diluents and supports may be employed in any number of methods. For example, in some embodiments a support (e.g., MgO, CaO, CaCO₃, SrCO₃) may be used in the form of a pellet or monolith (e.g., honeycomb) structure, and the catalysts may be impregnated or supported thereon. In other embodiments, a core/shell arrangement is provided and the support material may form part of the core or shell. For example, a core of MgO, CaO, CaCO₃ or SrCO₃ may be coated with a shell of any of the disclosed catalyst compositions.

In some embodiments, the diluent has a morphology selected from bulk (e.g., commercial grade), nano (nanowires, nanorods, nanoparticles, etc.) or combinations thereof.

In some embodiments, the diluent has none to moderate catalytic activity at the temperature the OCM catalyst is operated. In some other embodiments, the diluent has moderate to large catalytic activity at a temperature higher than the temperature the OCM catalyst is operated. In yet some other embodiments, the diluent has none to moderate catalytic activity at the temperature the OCM catalyst is operated and moderate to large catalytic activity at temperatures higher than the temperature the OCM catalyst is operated. Typical temperatures for operating an OCM reaction according to the present disclosure are 800° C. or lower, 750° C. or lower, 700° C. or lower, 650° C. or lower, 600° C. or lower and 550° C. or lower.

For example, CaCO₃ is a relatively good OCM catalyst at T>750° C. (50% selectivity, >20% conversion) but has essentially no activity below 700° C. Experiments performed in support of the present invention showed that dilution of Nd₂O₃ straight nanowires with CaCO₃ or SrCO₃ (bulk) showed no degradation of OCM performance and, in some cases, even better performance than the neat catalyst.

In some embodiments, the diluent portion in the catalyst/diluent mixture is 0.01%, 10%, 30%, 50%, 70%, 90% or 99.99% (weight percent) or any other value between 0.01% and 99.9%. In some embodiments, the dilution is performed with the OCM catalyst ready to go, e.g., after calcination. In some other embodiments, the dilution is performed prior to the final calcination of the catalyst, i.e., the catalyst and the diluent are calcined together. In yet some other embodiments, the dilution can be done during the synthesis as well, so that, for example, a mixed oxide is formed.

In some embodiments, the catalyst/diluent mixture comprises more than one catalyst and/or more than one diluent. In some other embodiments, the catalyst/diluent mixture is pelletized and sized, or made into shaped extrudates or deposited on a monolith or foam, or is used as it is. Methods of the invention include taking advantage of the very exothermic nature of OCM by diluting the catalyst with another catalyst that is (almost) inactive in the OCM reaction at the operating temperature of the first catalyst but active at higher temperature. In these methods, the heat generated by the hotspots of the first catalyst will provide the necessary heat for the second catalyst to become active.

For ease of illustration, the above description of catalytic materials often refers to OCM; however, such catalytic materials find utility in other catalytic reactions including but not limited to: oxidative dehydrogenation (ODH) of alkanes to their corresponding alkenes, selective oxidation of alkanes and alkenes and alkynes, oxidation of co, dry reforming of methane, selective oxidation of aromatics, Fischer-Tropsch, combustion of hydrocarbons, etc.

3. Preparation of Catalysts and Catalytic Materials

The catalysts can be prepared using any suitable method (e.g., such that the catalyst functions as an OCM catalyst). Suitable methods, which include using a bacteriophage template and other methods known in the art, are described in U.S. application Ser. No. 13/115,082 (U.S. Pub. No. 2012/0041246), Ser. No. 13/689,514 (U.S. Pub. No. 2013/0158322) and Ser. No. 13/689,611 (U.S. Pub. No. 2013/0165728), which applications are hereby incorporated by reference in their entireties.

In some embodiments, the nanowire catalysts can be synthesized in a solution phase in the absence of a template. Typically, a hydrothermal or sol gel approach can be used to create straight (i.e., ratio of effective length to actual length equal to one) and substantially single crystalline nanowires. As an example, nanowires comprising a metal oxide can be prepared by (1) forming nanowires of a metal oxide precursor (e.g., metal hydroxide) in a solution of a metal salt and an anion precursor; (2) isolating the nanowires of the metal oxide precursor; and (3) calcining the nanowires of the metal oxide precursor to provide nanowires of a corresponding metal oxide. In other embodiments (for example MgO nanowires), the synthesis goes through an intermediate which can be prepared as a nanowire and then converted into the desired product while maintaining its morphology. Optionally, the nanowires comprising a metal oxide can be doped according to methods described herein.

In other certain embodiments, nanowires comprising a core/shell structure are prepared in the absence of a biological template. Such methods may include, for example, preparing a nanowire comprising a first metal and growing a shell on the outersurface of this nanowire, wherein the shell comprises a second metal. The first and second metals may be the same or different.

In other aspects, a core/shell nanowire is prepared in the absence of a biological template. Such methods comprise preparing a nanowire comprising an inner core and an outer shell, wherein the inner core comprises a first metal, and the outer shell comprises a second metal, the method comprising:

-   -   a) preparing a first nanowire comprising the first metal; and     -   b) treating the first nanowire with a salt comprising the second         metal.

In some embodiments of the foregoing method, the method further comprises addition of a base to a solution obtained in step b). In yet other examples, the first metal and the second metal are different. In yet further embodiments, the salt comprising the second metal is a halide or a nitrate. In certain aspects it may be advantageous to perform one or more sequential additions of the salt comprising the second metal and a base. Such sequential additions help prevent non-selective precipitation of the second metal and favor conditions wherein the second metal nucleates on the surface of the first nanowire to form a shell of the second metal. Furthermore, the first nanowire may be prepared by any method, for example via a template directed method (e.g., phage).

As with template-directed syntheses, the synthetic conditions and parameters for direct synthesis (template free) of nanowires can also be adjusted to create diverse compositions and surface morphologies (e.g., crystal faces) and dopant levels. For example, variable synthetic parameters include: concentration ratios of metal and anions (e.g., hydroxide); reaction temperature; reaction time; sequence of adding anion and metal ions; pH; types of metal precursor salt; types of anion precursor; number of additions; the time that lapses between the additions of the metal salt and anion precursor, including, e.g., simultaneous (zero lapse) or sequential additions followed by respective incubation times for the metal salt and the anion precursor.

In addition, the choice of solvents or surfactants may influence the crystal growth of the nanowires, thereby generating different nanowire dimensions (including aspect ratios). For example, solvents such as ethylene glycol, poly(ethylene glycol), polypropylene glycol and poly(vinyl pyrrolidone) can serve to passivate the surface of the growing nanowires and facilitate a linear growth of the nanowire.

In some embodiments, nanowires can be prepared directly from the corresponding oxide. For example, metal oxides may be treated with halides, for example ammonium halides, to produce nanowires. Such embodiments find particular utility in the context of lanthanide oxides, for example La₂O₃, are particularly useful since the procedure is quite simple and economically efficient Nanowires comprising two or more metals and/or dopants may also be prepared according to these methods. Accordingly, in some embodiments at least one of the metal compounds is an oxide of a lanthanide element.

Accordingly, in one embodiment the present disclosure provides a method for preparing a nanowire in the absence of a biological template, the method comprising treating at least one metal compound with a halide. In certain embodiments, nanowires comprising more than one type of metal and/or one or more dopants can be prepared by such methods. For example, in one embodiment the method comprises treating two or more different metal compounds with a halide and the nanowire comprises two or more different metals. The nanowire may comprise a mixed metal oxide, metal oxyhalide, metal oxynitrate or metal sulfate.

In some other embodiments of the foregoing, the halide is in the form of an ammonium halide. In yet other embodiments, the halide is contacted with the metal compound in solution or in the solid state.

In certain embodiments, the method is useful for incorporation of one or more doping elements into a nanowire. For example, the method may comprise treating at least one metal compound with a halide in the presence of at least one doping element, and the nanowire comprises the least one doping element. In some aspects, the at least one doping element is present in the nanowire in an atomic percent ranging from 0.1 to 50 at %.

Other methods for preparation of nanowires in the absence of a biological template include preparing a hydroxide gel by reaction of at least one metal salt and a hydroxide base. For example, the method may further comprise aging the gel, heating the gel or combinations thereof. In certain other embodiments, the method comprises reaction of two or more different metal salts, and the nanowire comprises two or more different metals.

Doping elements may also be incorporated by using the hydroxide gel method described above, further comprising addition of at least one doping element to the hydroxide gel, and wherein the nanowire comprises the at least one doping element. For example, the at least one doping element may be present in the nanowire in an atomic percent ranging from 0.1 to 50 at %.

In some embodiments, metal oxide nanowires can be prepared by mixing a metal salt solution and an anion precursor so that a gel of a metal oxide precursor is formed. This method can work for cases where the typical morphology of the metal oxide precursor is a nanowire. The gel is thermally treated so that crystalline nanowires of the metal oxide precursor are formed. The metal oxide precursor nanowires are converted to metal oxide nanowires by calcination. This method can be especially useful for lanthanides and group 3 elements. In some embodiments, the thermal treatment of the gel is hydrothermal (or solvothermal) at temperatures above the boiling point of the reaction mixture and at pressures above ambient pressure, in other embodiments it's done at ambient pressure and at temperatures equal to or below the boiling point of the reaction mixture. In some embodiments the thermal treatment is done under reflux conditions at temperatures equal to the boiling point of the mixture. In some specific embodiments the anion precursor is a hydroxide, e.g., Ammonium hydroxide, sodium hydroxide, lithium hydroxide, tetramethyl ammonium hydroxide, and the like. In some other specific embodiments the metal salt is LnCl₃ (Ln=Lanthanide), in other embodiment the metal salt is Ln(NO₃)₃. In yet other embodiments, the metal salt is YCl₃, ScCl₃, Y(NO₃)₃, Sc(NO₃)₃. In some other embodiments, the metal precursor solution is an aqueous solution. In other embodiments, the thermal treatment is done at T=100° C. under reflux conditions.

This method can be used to make mixed metal oxide nanowires, by mixing at least two metal salt solutions and an anion precursor so that a mixed oxide precursor gel is formed. In such cases, the first metal may be a lanthanide or a group 3 element, and the other metals can be from other groups, including groups 1-14.

In some different embodiments, metal oxide nanowires can be prepared in a similar way as described above by mixing a metal salt solution and an anion precursor so that a gel of a metal hydroxide precursor is formed. This method works for cases where the typical morphology of the metal hydroxide precursor is a nanowire. The gel is treated so that crystalline nanowires of the metal hydroxide precursor are formed. The metal hydroxide precursor nanowires are converted to metal hydroxide nanowires by base treatment and finally converted to metal oxide nanowires by calcination. This method may be especially applicable for group 2 elements, for example Mg. In some specific embodiments, the gel treatment is a thermal treatment at temperatures in the range 50-100° C. followed by hydrothermal treatment. In other embodiments, the gel treatment is an aging step. In some embodiments, the aging step takes at least one day. In some specific embodiments, the metal salt solution is a concentrated metal chloride aqueous solution and the anion precursor is the metal oxide. In some more specific embodiments, the metal is Mg. In certain embodiments of the above, these methods can be used to make mixed metal oxide nanowires. In these embodiments, the first metal is Mg and the other metal can be any other metal of groups 1-14+Ln.

The catalysts and/or catalytic materials can be prepared according to any number of methods known in the art. For example, the catalysts and/or catalytic materials can be prepared after preparation of the individual components by mixing the individual components in their dry form, e.g., blend of powders, and optionally, ball milling can be used to reduce particle size and/or increase mixing. Each component can be added together or one after the other to form layered particles. Alternatively, the individual components can be mixed prior to calcination, after calcination or by mixing already calcined components with uncalcined components. The catalysts and/or catalytic materials may also be prepared by mixing the individual components in their dry form and optionally pressing them together into a “pill” followed by calcination to above 400° C.

The foregoing catalysts may be doped prior to, or after formation of the catalyst. In one embodiment, one or more metal salts are mixed to form a solution or a slurry which is dried and then calcined in a range of 400° C. to 900° C., or between 500° C. and 700° C. In another embodiment, the catalyst is formed first through calcination of one or more metal salt followed by contact with a solution comprising the doping element followed by drying and/or calcination between 300° C. and 800° C., or between 400° C. and 700° C.

In other examples, the catalysts and/or catalytic materials are prepared by mixing the individual components with one or more solvents into a solution, suspension or slurry. Optional mixing and/or ball milling can be used to maximize uniformity and reduce particle size. Examples of solvents useful in this context include, but are not limited to: water, alcohols, ethers, carboxylic acids, ketones, esters, amides, aldehydes, amines, alkanes, alkenes, alkynes, aromatics, etc. In other embodiments, the individual components are deposited on a supporting material such as silica, alumina, magnesia, activated carbon, and the like, or by mixing the individual components using a fluidized bed granulator. Combinations of any of the above methods may also be used.

The catalysts and/or catalytic materials may optionally comprise a dopant as described in more detail herein. In this respect, doping material(s) may be added during preparation of the individual components, after preparation of the individual components but before drying of the same, after the drying step but before calcinations or after calcination. If more than one doping material is used, each dopant can be added together or one after the other to form layers of dopants.

Doping material(s) may also be added as dry components and optionally ball milling can be used to increase mixing. In other embodiments, doping material(s) are added as a liquid (e.g., solution, suspension, slurry, etc.) to the dry individual catalyst components or to the blended catalytic material. The amount of liquid may optionally be adjusted for optimum wetting of the catalyst, which can result in optimum coverage of catalyst particles by doping material. Mixing and/or ball milling can also be used to maximize doping coverage and uniform distribution. Alternatively, doping material(s) are added as a liquid (e.g., solution, suspension, slurry, etc.) to a suspension or slurry of the catalyst in a solvent. Mixing and/or ball milling can be used to maximize doping coverage and uniform distribution. Incorporation of dopants can also be achieved using any of the methods described elsewhere herein.

As noted herein, an optional calcination step usually follows an optional drying step at T<200 C (typically 60-120 C) in a regular oven or in a vacuum oven. Calcination may be performed on the individual components of the catalysts and/or catalytic material or on the blended catalysts and/or catalytic material. Calcination is generally performed in an oven/furnace at a temperature higher than the minimum temperature at which at least one of the components decomposes or undergoes a phase transformation and can be performed in inert atmosphere (e.g., N₂, Ar, He, etc.), oxidizing atmosphere (air, O₂, etc.) or reducing atmosphere (H₂, H₂/N₂, H₂/Ar, etc.). The atmosphere may be a static atmosphere or a gas flow and may be performed at ambient pressure, at p<1 atm, in vacuum or at p>1 atm. High pressure treatment (at any temperature) may also be used to induce phase transformation including amorphous to crystalline. Calcinations may also be performed using microwave heating.

Calcination is generally performed in any combination of steps comprising ramp up, dwell and ramp down. For example, ramp to 500° C., dwell at 500° C. for 5 h, ramp down to RT. Another example includes ramp to 100° C., dwell at 100° C. for 2 h, ramp to 300° C., dwell at 300° C. for 4 h, ramp to 550° C., dwell at 550° C. for 4 h, ramp down to RT. Calcination conditions (pressure, atmosphere type, etc.) can be changed during the calcination. In some embodiments, calcination is performed before preparation of the blended catalytic material (i.e., individual components are calcined), after preparation of the blended catalytic material but before doping, after doping of the individual components or blended catalytic material. Calcination may also be performed multiple times, e.g., after catalyst preparation and after doping.

The catalytic materials may be incorporated into a reactor bed for performing any number of catalytic reactions (e.g., OCM, ODH and the like). Accordingly, in one embodiment the present disclosure provides a catalytic material as disclosed herein in contact with a reactor and/or in a reactor bed. For example, the reactor may be for performing an OCM reaction, may be a fixed bed reactor and may have a diameter greater than 1 inch. In this regard, the catalytic material may be packed neat (without diluents) or diluted with an inert material (e.g., sand, silica, alumina, etc.) The catalyst components may be packed uniformly forming a homogeneous reactor bed.

The particle size of the individual components within a catalytic material may also alter the catalytic activity, and other properties, of the same. Accordingly, in one embodiment, the catalyst is milled to a target average particle size or the catalyst powder is sieved to select a particular particle size. In some aspects, the catalyst powder may be pressed into pellets and the catalyst pellets can be optionally milled and or sieved to obtain the desired particle size distribution.

In some embodiments, the catalyst materials, alone or with binders and/or diluents, can be configured into larger aggregate forms, such as pellets, extrudates, or other aggregations of catalyst particles. For ease of discussion, such larger forms are generally referred to herein as “pellets”. Such pellets may optionally include a binder and/or support material; however, the present inventors have surprisingly found that the disclosed nanowires are particularly suited to use in the form of a pellet without a binder and/or support material. Accordingly, one embodiment of the disclosure provides a catalytic material in the absence of a binder. In this regard, the morphology of the disclosed nanowires (either bent or straight, etc.) is believed to contribute to the nanowires' ability to be pressed into pellets without the need for a binder. Catalytic materials without binders are simpler, less complex and cheaper than corresponding materials with binders and thus offer certain advantages.

In some instances, catalytic materials may be prepared using a “sacrificial binder” or support. Because of their special properties, the nanowires allow for preparation of catalytic material forms (e.g., pellets) without the use of a binder. A “sacrificial” binder can be used in order to create unique microporosity in pellets or extrudates. After removing the sacrificial binder, the structural integrity of the catalyst is ensured by the special binding properties of the nanowires and the resulting catalytic material has unique microporosity as a result of removing the binder. For example, in some embodiments a catalytic nanowire may be prepared with a binder and then the binder removed by any number of techniques (e.g., calcinations, acid erosion, etc.). This method allows for design and preparation of catalytic materials having unique microporosity (i.e., the microporosity is a function of size, etc., of the sacrificial binder). The ability to prepare different forms (e.g., pellets) of the nanowires without the use of binder is not only useful for preparation of catalytic materials from the nanowires, but also allows the nanowires to be used as support materials (or both catalytic and support material). Sacrificial binders and techniques useful in this regard include sacrificial cellulosic fibers or other organic polymers that can be easily removed by calcination, non-sacrificial binders and techniques useful in this regard include, colloidal oxide binders such as Ludox Silica or Nyacol colloidal zirconia that can also be added to strengthen the formed aggregate when needed. Sacrificial binders are added to increase macro-porosity (pores larger than 20 nm diameter) of the catalytic materials. Accordingly, in some embodiments the catalytic materials comprise pores greater than 20 nm in diameter, greater than 50 nm in diameter, greater than 75 nm in diameter, greater than 100 nm in diameter or greater than 150 nm in diameter.

Catalytic materials also include any of the disclosed catalysts disposed on or adhered to a solid support. For example, the catalysts may be adhered to the surface of a monolith support. As with the binder-less materials discussed above, in these embodiments the catalysts may be adhered to the surface of the monolith in the absence of a binder due to their unique morphology and packing properties. Monoliths include honeycomb-type structures, foams and other catalytic support structures derivable by one skilled in the art. In one embodiment, the support is a honeycomb matrix formed from silicon carbide, and the support further comprises the disclosed catalyst disposed on the surface.

As the OCM reaction is very exothermic, it can be desirable to reduce the rate of conversion per unit volume of reactor in order to avoid run away temperature rise in the catalyst bed that can result in hot spots affecting performance and catalyst life. One way to reduce the OCM reaction rate per unit volume of reactor is to spread the active catalyst onto an inert support with interconnected large pores as in ceramic or metallic foams (including metal alloys having reduced reactivity with hydrocarbons under OCM reaction conditions) or having arrays of channel as in honeycomb structured ceramic or metal assembly.

In one embodiment, a catalytic material comprising a catalyst as disclosed herein supported on a structured support is provided. Examples of such structure supports include, but are not limited to, metal foams, Silicon Carbide or Alumina foams, corrugated metal foil arranged to form channel arrays, extruded ceramic honeycomb, for example Cordierite (available from Corning or NGK ceramics, USA), Silicon Carbide or Alumina.

Active catalyst loading on the structured support ranges from 1 to 500 mg per ml of support component, for example from 5 to 100 mg per ml of structure support. Cell densities on honeycomb structured support materials may range from 100 to 900 CPSI (cell per square inch), for example 200 to 600 CPSI. Foam densities may range from 10 to 100 PPI (pore per inch), for example 20 to 60 PPI.

In other embodiments, the exotherm of the OCM reaction may be at least partially controlled by blending the active catalytic material with catalytically inert material, and pressing or extruding the mixture into shaped pellets or extrudates. In some embodiments, these mixed particles may then be loaded into a pack-bed reactor. The Extrudates or pellets comprise between 30% to 70% pore volume with 5% to 50% active catalyst weight fraction. Useful inert materials in this regard include, but are not limited to MgO, CaO, Al₂O₃, SiC and cordierite.

In addition to reducing the potential for hot spots within the catalytic reactor, another advantage of using a structured ceramic with large pore volume as a catalytic support is reduced flow resistance at the same gas hourly space velocity versus a pack-bed containing the same amount of catalyst.

Yet another advantage of using such supports is that the structured support can be used to provide features difficult to obtain in a pack-bed reactor. For example the support structure can improve mixing or enabling patterning of the active catalyst depositions through the reactor volume. Such patterning can consist of depositing multiple layers of catalytic materials on the support in addition to the OCM active component in order to affect transport to the catalyst or combining catalytic functions as adding O₂-ODH activity, CO₂-OCM activity or CO₂-ODH activity to the system in addition to O₂-OCM active material. Another patterning strategy can be to create bypass within the structure catalyst essentially free of active catalyst to limit the overall conversion within a given supported catalyst volume.

Yet another advantage is reduced heat capacity of the bed of the structured catalyst over a pack bed a similar active catalyst loading therefore reducing startup time.

Nanowire shaped catalysts are particularly well suited for incorporation into pellets or extrudates or deposition onto structured supports. Nanowire aggregates forming a mesh type structure can have good adhesion onto rough surfaces.

The mesh like structure can also provide improved cohesion in composite ceramic improving the mechanical properties of pellets or extrudates containing the nanowire shaped catalyst particles.

Alternatively, such nanowire on support or in pellet form approaches can be used for other reactions besides OCM, such as ODH, dry methane reforming, FT, and all other catalytic reactions.

In yet another embodiment, the catalysts are packed in bands forming a layered reactor bed. Each layer is composed by either a catalyst of a particular type, morphology or size or a particular blend of catalysts. In one embodiment, the catalysts blend may have better sintering properties, i.e., lower tendency to sinter, then a material in its pure form. Better sintering resistance is expected to increase the catalyst's lifetime and improve the mechanical properties of the reactor bed.

In yet other embodiments, the disclosure provides a catalytic material comprising one or more different catalysts. The catalysts may be a nanowire as disclosed herein and a different catalyst for example a bulk catalysts. Mixtures of two or more nanowire catalysts are also contemplated. The catalytic material may comprise a catalyst, for example a nanowire catalyst, having good OCM activity and a catalyst having good activity in the ODH reaction. Either one or both of these catalysts may be nanowires as disclosed herein.

One skilled in the art will recognize that various combinations or alternatives of the above methods are possible, and such variations are also included within the scope of the present disclosure.

4. Structure/Physical Characteristics of the Disclosed Catalysts

Typically, a catalytic material described herein comprises a plurality of metal oxide particles. In certain embodiments, the catalytic material may further comprise a support material. The total surface area per gram of a catalytic material may have an effect on the catalytic performance. Pore size distribution may affect the catalytic performance as well. Surface area and pore size distribution of the catalytic material can be determined by BET (Brunauer, Emmett, Teller) measurements. BET techniques utilize nitrogen adsorption at various temperatures and partial pressures to determine the surface area and pore sizes of catalysts. BET techniques for determining surface area and pore size distribution are well known in the art.

In some embodiments the catalytic material comprises a surface area of between 0.1 and 100 m²/g, between 1 and 100 m²/g, between 1 and 50 m²/g, between 1 and 20 m²/g, between 1 and 10 m²/g, between 1 and 5 m²/g, between 1 and 4 m²/g, between 1 and 3 m²/g, or between 1 and 2 m²/g.

Additional structural properties of the catalysts and catalytic materials are described in U.S. application Ser. No. 13/115,082 (U.S. Pub. No. 2012/0041246), Ser. No. 13/689,514 (U.S. Pub. No. 2013/0158322) and Ser. No. 13/689,611 (U.S. Pub. No. 2013/0165728), which applications are hereby incorporated by reference in their entireties.

Catalytic Reactions

The present disclosure provides heterogeneous catalysts having better catalytic properties than known catalysts. The catalysts disclosed herein are useful in any number of reactions catalyzed by a heterogeneous catalyst. Examples of reactions wherein the disclosed catalysts may be employed are disclosed in Farrauto and Bartholomew, “Fundamentals of Industrial Catalytic Processes” Blackie Academic and Professional, first edition, 1997, which is hereby incorporated in its entirety. Other non-limiting examples of reactions wherein the catalysts may be employed include: the oxidative coupling of methane (OCM) to ethane and ethylene; oxidative dehydrogenation (ODH) of alkanes to the corresponding alkenes, for example oxidative dehydrogenation of ethane or propane to ethylene or propylene, respectively; selective oxidation of alkanes, alkenes, and alkynes; oxidation of CO, dry reforming of methane, selective oxidation of aromatics; Fischer-Tropsch, hydrocarbon cracking; combustion of hydrocarbons and the like. Reactions catalyzed by the disclosed catalysts are discussed in more detail below. While an embodiment of the invention is described in greater detail below in the context of the OCM reaction and other reactions described herein, the catalysts are not in any way limited to the particularly described reactions.

The disclosed catalysts are generally useful in methods for converting a first carbon-containing compound (e.g., a hydrocarbon, CO or CO₂) to a second carbon-containing compound. In some embodiments the methods comprise contacting a disclosed catalyst, or material comprising the same, with a gas comprising a first carbon-containing compound and an oxidant to produce a second carbon-containing compound. In some embodiments, the first carbon-containing compound is a hydrocarbon, CO, CO₂, methane, ethane, propane, hexane, cyclohexane, octane or combinations thereof. In other embodiments, the second carbon-containing compound is a hydrocarbon, CO, CO₂, ethane, ethylene, propane, propylene, hexane, hexene, cyclohexane, cyclohexene, bicyclohexane, octane, octene or hexadecane. In some embodiments, the oxidant is oxygen, ozone, nitrous oxide, nitric oxide, water, carbon dioxide or combinations thereof.

In other embodiments of the foregoing, the method for conversion of a first carbon-containing compound to a second carbon-containing compound is performed at a temperature below 100° C., below 200° C., below 300° C., below 400° C., below 500° C., below 550° C., below 600° C., below 700° C., below 800° C., below 900° C. or below 1000° C. In certain embodiments, the method is OCM and the method is performed at a temperature below 600° C., below 700° C., below 800° C., or below 900° C. In other embodiments, the method for conversion of a first carbon-containing compound to a second carbon-containing compound is performed at a pressure above 0.5 atm, above 1 atm, above 2 atm, above 5 atm, above 10 atm, above 15 atm above 25 atm or above 50 atm.

The catalytic reactions described herein can be performed using standard laboratory equipment known to those of skill in the art, for example as described in U.S. Pat. No. 6,350,716, which is incorporated herein in its entirety.

As noted above, the catalysts disclosed herein have better catalytic activity than a corresponding undoped catalyst. In some embodiments, the selectivity, yield, conversion, or combinations thereof, of a reaction catalyzed by the catalysts is better than the selectivity, yield, conversion, or combinations thereof, of the same reaction catalyzed by a corresponding undoped catalyst under the same conditions. For example, in some embodiments, the catalysts are doped bulk catalysts or nanowire catalysts (doped or undoped) and the catalysts possess a catalytic activity such that conversion of reactant to product in a reaction catalyzed by the catalyst is at least 1.1 times, at least 1.25 times, at least 1.5 times, at least 2.0 times, at least 3.0 times or at least 4.0 times the yield of product in the same reaction under the same conditions but catalyzed by a corresponding catalyst. As used herein a “corresponding catalyst” refers to:

1) an undoped bulk catalyst (i.e., a catalyst comprising the same base material but different or no dopants or different ratios or concentrations of the same dopants) when the comparison is to a doped bulk catalyst of the invention;

2) a bulk catalyst (i.e., a catalyst prepared from bulk material having the same chemical composition as the nanowire, including any dopants) when the comparison is to a doped or undoped nanowire catalyst of the invention; or

3) an undoped nanowire catalyst when the comparison is to a doped nanowire of the invention.

For purpose of clarity, it should be noted that this comparison (and others throughout the application) is made to an undoped bulk catalyst when the catalysts are doped bulk catalysts and to a corresponding bulk catalyst when the catalysts are nanowire catalysts.

In other embodiments, the catalysts are doped bulk catalysts or nanowire catalysts (doped or undoped) and the catalysts possess a catalytic activity such that selectivity for product in a reaction catalyzed by the catalyst is at least 1.1 times, at least 1.25 times, at least 1.5 times, at least 2.0 times, at least 3.0 times or at least 4.0 times the yield of product in the same reaction under the same conditions but catalyzed by a corresponding catalyst.

In yet other embodiments, the catalysts are doped bulk catalysts or nanowire catalysts (doped or undoped) and the catalysts possess a catalytic activity such that yield of product in a reaction catalyzed by the catalyst is at least 1.1 times, at least 1.25 times, at least 1.5 times, at least 2.0 times, at least 3.0 times or at least 4.0 times the yield of product in the same reaction under the same conditions but catalyzed by a corresponding catalyst. In yet other embodiments, the catalysts are doped bulk catalysts or nanowire catalysts (doped or undoped) and the catalysts possess a catalytic activity such that the activation temperature of a reaction catalyzed by the catalyst is at least 25° C. lower, at least 50° C. lower, at least 75° C. lower, or at least 100° C. lower than the temperature of the same reaction under the same conditions but catalyzed by a corresponding catalyst. In certain reactions (e.g., OCM), production of unwanted oxides of carbon (e.g., CO and CO₂) is a problem that reduces overall yield of desired product and results in an environmental liability. Accordingly, in one embodiment the present disclosure addresses this problem and provides catalysts with a catalytic activity such that the selectivity for CO and/or CO₂ in a reaction catalyzed by the catalysts is less than the selectivity for CO and/or CO₂ in the same reaction under the same conditions but catalyzed by an undoped catalyst. Accordingly, in one embodiment, the present disclosure provides a doped bulk catalysts or nanowire catalysts (doped or undoped) and the catalysts possess a catalytic activity such that selectivity for CO_(x), wherein x is 1 or 2, in a reaction catalyzed by the catalyst is less than at least 0.9 times, less than at least 0.8 times, less than at least 0.5 times, less than at least 0.2 times or less than at least 0.1 times the selectivity for CO_(x) in the same reaction under the same conditions but catalyzed by a corresponding catalyst.

In some embodiments, the absolute selectivity, yield, conversion, or combinations thereof, of a reaction catalyzed by the catalysts disclosed herein is better than the absolute selectivity, yield, conversion, or combinations thereof, of the same reaction under the same conditions but catalyzed by a corresponding catalyst. For example, in some embodiments the yield (e.g., C2+ yield) of desired product(s) in a reaction catalyzed by the catalysts is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%. In some embodiments, the reaction is OCM and the yield of product is greater than 10%, greater than 20%, greater than 30% or greater than 40%. In other embodiments, the selectivity for product (e.g., C2+ selectivity) in a reaction catalyzed by the catalysts is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%. In other embodiments, the conversion (e.g., methane conversion) of reactant to product in a reaction catalyzed by the catalysts is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In certain embodiments wherein the catalysts are nanowires, the morphology of the nanowires is expected to provide for improved mixing properties for the nanowires compared to standard colloidal (e.g., bulk) catalyst materials. The improved mixing properties are expected to improve the performance of any number of catalytic reactions, for example, in the area of transformation of heavy hydrocarbons where transport and mixing phenomena are known to influence the catalytic activity. In other reactions, the shape of the nanowires is expected to provide for good blending, reduce settling, and provide for facile separation of any solid material.

In some other chemical reactions, the nanowires are useful for absorption and/or incorporation of a reactant used in chemical looping. For example, the nanowires find utility as NO_(x) traps, in unmixed combustion schemes, as oxygen storage materials, as CO₂ sorption materials (e.g., cyclic reforming with high H₂ output) and in schemes for conversion of water to H₂.

1. Oxidative Coupling of Methane (OCM)

As noted above, the present disclosure provides catalysts having catalytic activity and related approaches to catalyst design and preparation for improving the yield, selectivity and/or conversion of any number of catalyzed reactions, including the OCM reaction. Reactors useful in practice of the OCM methods described herein are known in the art and are described in PCT Pub. No. WO 2013/177433, which application is hereby incorporated by reference in its entirety. As mentioned above, there exists a tremendous need for catalyst technology capable of addressing the conversion of methane into high value chemicals (e.g., ethylene and products prepared therefrom) using a direct route that does not go through syngas. Accomplishing this task will dramatically impact and redefine a non-petroleum based pathway for feedstock manufacturing and liquid fuel production yielding reductions in GHG emissions, as well as providing new fuel sources.

Ethylene has the largest carbon footprint compared to all industrial chemical products in part due to the large total volume consumed into a wide range of downstream important industrial products including plastics, surfactants and pharmaceuticals. In 2008, worldwide ethylene production exceeded 120 M metric tons while growing at a robust rate of 4% per year. The United States represents the largest single producer at 28% of the world capacity. Ethylene is primarily manufactured from high temperature cracking of naphtha (e.g., oil) or ethane that is separated from natural gas. The true measurement of the carbon footprint can be difficult as it depends on factors such as the feedstock and the allocation as several products are made and separated during the same process. However, some general estimates can be made based on published data.

Cracking consumes a significant portion (about 65%) of the total energy used in ethylene production and the remainder is for separations using low temperature distillation and compression. The total tons of CO₂ emission per ton of ethylene are estimated at between 0.9 to 1.2 from ethane cracking and 1 to 2 from naphtha cracking. Roughly, 60% of ethylene produced is from naphtha, 35% from ethane and 5% from others sources (Ren, T.; Patel, M. Res. Conserv. Recycl. 53:513, 2009). Therefore, based on median averages, an estimated amount of CO₂ emissions from the cracking process is 114M tons per year (based on 120M tons produced). Separations would then account for an additional 61M tons CO₂ per year.

The catalysts of this disclosure provide an alternative to the need for the energy intensive cracking step. Additionally, because of the high selectivity of the catalysts, downstream separations are dramatically simplified, as compared to cracking which yields a wide range of hydrocarbon products. The reaction is also exothermic so it can proceed via an autothermal process mechanism. Overall, it is estimated that up to a potential 75% reduction in CO₂ emission compared to conventional methods could be achieved. This would equate to a reduction of one billion tons of CO₂ over a ten-year period and would save over 1M barrels of oil per day.

The catalysts of this disclosure also permit converting ethylene into liquid fuels such as gasoline or diesel, given ethylene's high reactivity and numerous publications demonstrating high yield reactions, in the lab setting, from ethylene to gasoline and diesel. On a life cycle basis from well to wheel, recent analysis of methane to liquid (MTL) using F-T process derived gasoline and diesel fuels has shown an emission profile approximately 20% greater to that of petroleum based production (based on a worst case scenario) (Jaramillo, P., Griffin, M., Matthews, S., Env. Sci. Tech 42:7559, 2008). In the model, the CO₂ contribution from plant energy was a dominating factor at 60%. Thus, replacement of the cracking and F-T process would be expected to provide a notable reduction in net emissions, and could be produced at lower CO₂ emissions than petroleum based production.

Furthermore, a considerable portion of natural gas is found in regions that are remote from markets or pipelines. Most of this gas is flared, re-circulated back into oil reservoirs, or vented given its low economic value. The World Bank estimates flaring adds 400M metric tons of CO₂ to the atmosphere each year as well as contributing to methane emissions. The catalysts of this disclosure also provide economic and environmental incentive to stop flaring. Also, the conversion of methane to fuel has several environmental advantages over petroleum-derived fuel. Natural gas is the cleanest of all fossil fuels, and it does not contain a number of impurities such as mercury and other heavy metals found in oil. Additionally, contaminants including sulfur are also easily separated from the initial natural gas stream. The resulting fuels burn much cleaner with no measurable toxic pollutants and provide lower emissions than conventional diesel and gasoline in use today.

In view of their wide range of applications, the catalysts (e.g., bulk and/or nanowires) of this disclosure can be used to not only selectively activate alkanes, but also to activate other classes of inert unreactive bonds, such as C—F, C—Cl or C—O bonds. This has importance, for example, in the destruction of man-made environmental toxins such as CFCs, PCBs, dioxins and other pollutants. Accordingly, while the invention is described in greater detail below in the context of the OCM reaction and the other reactions described herein, the nanowire catalysts are not in any way limited to this or any other particular reaction.

The selective, catalytic oxidative coupling of methane to ethylene (i.e., the OCM reaction) is shown by the following reaction (1): 2CH₄+O₂→CH₂CH₂+2H₂O  (1) This reaction is exothermic (Heat of Reaction −67 kcals/mole) and usually occurs at very high temperatures (>700° C.). During this reaction, it is believed that the methane (CH₄) is first oxidatively coupled into ethane (C₂H₆), and subsequently the ethane (C₂H₆) is oxidatively dehydrogenated into ethylene (C₂H₄). Because of the high temperatures used in the reaction, it has been suggested that the ethane is produced mainly by the coupling in the gas phase of the surface-generated methyl (CH₃) radicals. Reactive metal oxides (oxygen type ions) are apparently required for the activation of CH₄ to produce the CH₃ radicals. The yield of C₂H₄ and C₂H₆ is limited by further reactions in the gas phase and to some extent on the catalyst surface. A few of the possible reactions that occur during the oxidation of methane are shown below as reactions (2) through (8): CH₄→CH₃ radical  (2) CH₃ radical→C₂H₆  (3) CH₃ radical+2.5O₂→CO₂+1.5H₂O  (4) C₂H₆→C₂H₄+H₂  (5) C₂H₆+0.5O₂→C₂H₄+H₂O  (6) C₂H₄+3O₂→2CO₂+2H₂O  (7) CH₃ radical+C_(x)H_(y)+O₂→Higher HC's−Oxidation/CO₂+H₂O  (8)

With conventional heterogeneous catalysts and reactor systems, the reported performance is generally limited to <25% CH₄ conversion at <80% combined C2+ selectivity with the performance characteristics of high selectivity at low conversion, or the low selectivity at high conversion. In contrast, the catalysts of this disclosure are highly active and can optionally operate at a much lower temperature. In one embodiment, the catalysts disclosed herein enable efficient conversion of methane to ethylene in the OCM reaction at temperatures less than when other known catalysts are used. For example, in one embodiment, the catalysts disclosed herein enable efficient conversion (i.e., high yield, conversion, and/or selectivity) of methane to ethylene at temperatures of less than 900° C., less than 800° C., less than 700° C., less than 600° C., less than 550° C., or less than 500° C. In other embodiments, the use of staged oxygen addition, designed heat management, rapid quench and/or advanced separations may also be employed.

Typically, the OCM reaction is run in a mixture of oxygen and nitrogen or other inert gas. Such gasses are expensive and increase the overall production costs associated with preparation of ethylene or ethane from methane. However, the present inventors have now discovered that such expensive gases are not required and high yield, conversion, selectivity, etc., can be obtained when air is used as the gas mixture instead of pre-packaged and purified sources of oxygen and other gases. Accordingly, in one embodiment the disclosure provides a method for performing the OCM reaction using air as the oxidizer source.

Accordingly, one embodiment of the present disclosure is a method for the preparation of ethane and/or ethylene, the method comprising converting methane to ethane and/or ethylene in the presence of a catalytic material, wherein the catalytic material comprises at least one catalyst as disclosed herein.

Accordingly, in one embodiment a stable, very active, high surface area, multifunctional catalyst is disclosed having active sites that are isolated and precisely engineered with the catalytically active metal centers/sites in the desired proximity (see, e.g., FIG. 1) for facilitating the OCM reaction, as well as other reactions.

The exothermic heats of reaction (free energy) follow the order of reactions depicted above and, because of the proximity of the active sites, will mechanistically favor ethylene formation while minimizing complete oxidation reactions that form CO and CO₂. Representative catalyst compositions useful for the OCM reaction include, but are not limited to the catalyst compositions described herein, including both bulk and nanowire morphologies.

As noted above, the presently disclosed catalysts comprise a catalytic performance better than corresponding catalysts, for example in one embodiment the catalytic performance of the catalysts in the OCM reaction is better than the catalytic performance of a corresponding catalyst. In this regard, various performance criteria may define the “catalytic performance” of the catalysts in the OCM (and other reactions). In one embodiment, catalytic performance is defined by C2+ selectivity in the OCM reaction, and the C2+ selectivity of the catalysts in the OCM reaction is >5%, >10%, >15%, >20%, >25%, >30%, >35%, >40%, >45%, >50%, >55%, >60%, >65%, >70%, >75% or >80%.

Other important performance parameters used to measure the catalysts' catalytic performance in the OCM reaction are selected from single pass methane conversion percentage (i.e., the percent of methane converted on a single pass over the catalyst or catalytic bed, etc.), reaction inlet gas temperature, reaction operating temperature, total reaction pressure, methane partial pressure, gas-hour space velocity (GHSV), O₂ source, catalyst stability and ethylene to ethane ratio. In certain embodiments, improved catalytic performance is defined in terms of the catalysts' improved performance (relative to a corresponding catalyst) with respect to at least one of the foregoing performance parameters.

The reaction inlet gas temperature in an OCM reaction catalyzed by the disclosed catalysts can generally be maintained at a lower temperature, while maintaining better performance characteristics (e.g., conversion, C2+ yield, C2+ selectivity and the like) compared to the same reaction catalyzed by a corresponding undoped catalyst under the same reaction conditions. In certain embodiments, the inlet gas temperature in an OCM reaction catalyzed by the disclosed catalysts is <700° C., <675° C., <650° C., <625° C., <600° C., <593° C., <580° C., <570° C., <560° C., <550° C., <540° C., <530° C., <520° C., <510° C., <500° C., <490° C., <480° C. or even <470° C.

The reaction operating temperature in an OCM reaction catalyzed by the disclosed catalysts can generally be maintained at a lower temperature, while maintaining better performance characteristics compared to the same reaction catalyzed by a corresponding catalyst under the same reaction conditions. In certain embodiments, the reaction operating temperature (i.e., outlet temperature) in an OCM reaction catalyzed by the disclosed catalysts is <950° C., <925° C., <900° C., <875° C., <850° C., <825° C., <800° C., <775° C., <750° C., <725° C., <700° C., <675° C., <650° C., <625° C., <600° C., <593° C., <580° C., <570° C., <560° C., <550° C., <540° C., <530° C., <520° C., <510° C., <500° C., <490° C., <480° C., <470° C., <460° C., <450° C., <440° C., <430° C., <420° C., <410° C., <400° C.

The single pass methane conversion in an OCM reaction catalyzed by the catalysts is also generally better compared to the single pass methane conversion in the same reaction catalyzed by a corresponding catalyst under the same reaction conditions. For single pass methane conversion it is preferably >5%, >10%, >15%, >20%, >25%, >30%, >35%, >40%, >45%, >50%, >55%, >60%, >65%, >70%, >75%, or even >80%.

In certain embodiments, the inlet reaction pressure in an OCM reaction catalyzed by the catalysts is >1 atm, >1.1 atm, >1.2 atm, >1.3 atm, >1.4 atm, >1.5 atm, >1.6 atm, >1.7 atm, >1.8 atm, >1.9 atm, >2 atm, >2.1 atm, >2.1 atm, >2.2 atm, >2.3 atm, >2.4 atm, >2.5 atm, >2.6 atm, >2.7 atm, >2.8 atm, >2.9 atm, >3.0 atm, >3.5 atm, >4.0 atm, >4.5 atm, >5.0 atm, >5.5 atm, >6.0 atm, >6.5 atm, >7.0 atm, >7.5 atm, >8.0 atm, >8.5 atm, >9.0 atm, >10.0 atm, >11.0 atm, >12.0 atm, >13.0 atm, >14.0 atm, >15.0 atm, >16.0 atm, >17.0 atm, >18.0 atm, >19.0 atm or >20.0 atm.

In certain other embodiments, the total reaction pressure in an OCM reaction catalyzed by the catalysts ranges from about 1 atm to about 16 atm, from about 1 atm to about 11 atm, from about 1 atm to about 9 atm, from about 1 atm to about 7 atm, from about 1 atm to about 5 atm, from about 1 atm to about 3 atm or from about 1 atm to about 2 atm.

In some embodiments, the methane partial pressure in an OCM reaction catalyzed by the catalysts is >0.3 atm, >0.4 atm, >0.5 atm, >0.6 atm, >0.7 atm, >0.8 atm, >0.9 atm, >1 atm, >1.1 atm, >1.2 atm, >1.3 atm, >1.4 atm, >1.5 atm, >1.6 atm, >1.7 atm, >1.8 atm, >1.9 atm, >2.0 atm, >2.1 atm, >2.2 atm, >2.3 atm, >2.4 atm, >2.5 atm, >2.6 atm, >2.7 atm, >2.8 atm, >2.9 atm, >3.0 atm, >3.5 atm, >4.0 atm, >4.5 atm, >5.0 atm, >5.5 atm, >6.0 atm, >6.5 atm, >7.0 atm, >7.5 atm, >8.0 atm, >8.5 atm, >9.0 atm, >10.0 atm, >11.0 atm, >12.0 atm, >13.0 atm, >14.0 atm, >15.0 atm, >16.0 atm, >17.0 atm, >18.0 atm, >19.0 atm or >20.0 atm.

In some embodiments, the GSHV in an OCM reaction catalyzed by the catalysts is >10,000/hr, >15,000/hr, >20,000/hr, >30,000/hr, >50,000/hr, >75,000/hr, >100,000/hr, >120,000/hr, >130,000/hr, >150,000/hr, >200,000/hr, >250,000/hr, >300,000/hr, >350,000/hr, >400,000/hr, >450,000/hr, >500,000/hr, >750,000/hr, >1,000,000/hr, >2,000,000/hr, >3,000,000/hr, >4,000,000/hr.

In contrast to other OCM reactions, the present inventors have discovered that OCM reactions catalyzed by the disclosed catalysts can be performed (and still maintain high C2+ yield, C2+ selectivity, conversion, etc.) using O₂ sources other than pure O₂. For example, in some embodiments the O₂ source in an OCM reaction catalyzed by the disclosed catalysts is air, oxygen enriched air, pure oxygen, oxygen diluted with nitrogen (or another inert gas) or oxygen diluted with CO₂. In certain embodiments, the O₂ source is O₂ diluted by >99%, >98%, >97%, >96%, >95%, >94%, >93%, >92%, >91%, >90%, >85%, >80%, >75%, >70%, >65%, >60%, >55%, >50%, >45%, >40%, >35%, >30%, >25%, >20%, >15%, >10%, >9%, >8%, >7%, >6%, >5%, >4%, >3%, >2% or >1% with CO₂ or an inert gas, for example nitrogen.

The disclosed catalysts are also very stable under conditions required to perform any number of catalytic reactions, for example the OCM reaction. The stability of the catalysts is defined as the length of time a catalyst will maintain its catalytic performance without a significant decrease in performance (e.g., a decrease >20%, >15%, >10%, >5%, or greater than 1% in C2+ yield, C2+ selectivity or conversion, etc.). In some embodiments, the disclosed catalysts have stability under conditions required for the OCM reaction of >1 hr, >5 hrs, >10 hrs, >20 hrs, >50 hrs, >80 hrs, >90 hrs, >100 hrs, >150 hrs, >200 hrs, >250 hrs, >300 hrs, >350 hrs, >400 hrs, >450 hrs, >500 hrs, >550 hrs, >600 hrs, >650 hrs, >700 hrs, >750 hrs, >800 hrs, >850 hrs, >900 hrs, >950 hrs, >1,000 hrs, >2,000 hrs, >3,000 hrs, >4,000 hrs, >5,000 hrs, >6,000 hrs, >7,000 hrs, >8,000 hrs, >9,000 hrs, >10,000 hrs, >11,000 hrs, >12,000 hrs, >13,000 hrs, >14,000 hrs, >15,000 hrs, >16,000 hrs, >17,000 hrs, >18,000 hrs, >19,000 hrs, >20,000 hrs, >1 yrs, >2 yrs, >3 yrs, >4 yrs or >5 yrs.

In some embodiments, the ratio of ethylene to ethane in an OCM reaction catalyzed by the catalysts is better than the ratio of ethylene to ethane in an OCM reaction catalyzed by a corresponding undoped catalyst under the same conditions. In some embodiments, the ratio of ethylene to ethane in an OCM reaction catalyzed by the catalysts is >0.3, >0.4, >0.5, >0.6, >0.7, >0.8, >0.9, >1, >1.1, >1.2, >1.3, >1.4, >1.5, >1.6, >1.7, >1.8, >1.9, >2.0, >2.1, >2.2, >2.3, >2.4, >2.5, >2.6, >2.7, >2.8, >2.9, >3.0, >3.5, >4.0, >4.5, >5.0, >5.5, >6.0, >6.5, >7.0, >7.5, >8.0, >8.5, >9.0, >9.5, >10.0.

As noted above, the OCM reaction employing known catalysts suffers from poor yield, selectivity, or conversion. In contrast, the presently disclosed catalysts possess a catalytic activity in the OCM reaction such that the yield, selectivity, and/or conversion is better than when the OCM reaction is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the conversion of methane to ethylene in the oxidative coupling of methane reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the conversion of methane to ethylene compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of methane to ethylene in an OCM reaction catalyzed by the catalysts is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the yield of ethylene in the oxidative coupling of methane reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the yield of ethylene compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of methane to ethylene in an OCM reaction catalyzed by the catalytic materials is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%. In some embodiments the yield of ethylene in an OCM reaction catalyzed by the catalysts is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In certain embodiments, the catalysts possess a catalytic activity in the OCM reaction such that the yield, selectivity, and/or conversion is better than when the OCM reaction is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the conversion of methane in the oxidative coupling of methane reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the conversion of methane compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of methane in an OCM reaction catalyzed by the catalyst is greater than 10%, greater than 15%, greater than 20%, greater than 25%, greater than 30% greater than 40%, greater than 50%, greater than 75% or greater than 90%. In some embodiments the conversion of methane is determined when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less, 700° C. or less, 650° C. or less or even 600° C. or less. The conversion of methane may also be determined based on a single pass of a gas comprising methane over the catalyst or may be determined based on multiple passes over the catalyst.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the C2+ yield in the oxidative coupling of methane reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the C2+ yield compared to the same reaction under the same conditions but performed with a corresponding catalyst. In some embodiments the C2+ yield in an OCM reaction catalyzed by the catalyst is greater than 10%, greater than 15%, greater than 20%, greater than 25%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%. In some embodiments the C2+ yield is determined when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less, 700° C. or less, 650° C. or less or even 600° C. or less. The C2+ yield may also be determined based on a single pass of a gas comprising methane over the catalyst or may be determined based on multiple passes over the catalyst.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the C2+ selectivity in the oxidative coupling of methane reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the C2+ selectivity compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the C2+ selectivity in an OCM reaction catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 40%, greater than 50%, greater than 60%, greater than 65%, greater than 75%, or greater than 90%. In some embodiments the C2+ selectivity is determined when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less, 700° C. or less, 650° C. or less or even 600° C. or less. The C2+ selectivity may also be determined based on a single pass of a gas comprising methane over the catalyst or may be determined based on multiple passes over the catalyst.

In another embodiment, the disclosure provides a catalyst having a catalytic activity in the OCM reaction such that the nanowire has the same catalytic activity (i.e., same selectivity, conversion or yield), but at a lower temperature, compared to a corresponding catalyst. In some embodiments the catalytic activity of the catalysts in the OCM reaction is the same as the catalytic activity of a corresponding catalyst, but at a temperature of at least 20° C. less. In some embodiments the catalytic activity of the catalysts in the OCM reaction is the same as the catalytic activity of a corresponding catalyst, but at a temperature of at least 50° C. less. In some embodiments the catalytic activity of the catalysts in the OCM reaction is the same as the catalytic activity of a corresponding catalyst, but at a temperature of at least 100° C. less. In some embodiments the catalytic activity of the catalysts in the OCM reaction is the same as the catalytic activity of a corresponding catalyst, but at a temperature of at least 200° C. less.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the selectivity for CO or CO₂ in the oxidative coupling of methane reaction is less than at least 0.9 times, 0.8 times, 0.5 times, 0.2 times, or 0.1 times the selectivity for CO or CO₂ compared to the same reaction under the same conditions but performed with a corresponding catalyst.

In other embodiments, the above selectivity, conversion and yield values are determined at a temperature of less than 850° C., less than 800° C., less than 750° C., less than 700° C. or less than 650° C.

In some other embodiments, a method for converting methane into ethane and/or ethylene comprising use of catalyst mixture comprising two or more catalysts is provided. For example, the catalyst mixture may be a mixture of a catalyst having good OCM activity and a catalyst having good ODH activity. Catalysts suitable for such uses are described in more detail above.

Typically, the OCM reaction is run in a mixture of oxygen and nitrogen or other inert gas. Such gasses are expensive and increase the overall production costs associated with preparation of ethylene or ethane from methane. However, the present inventors have now discovered that such expensive gases are not required and high yield, conversion, selectivity, etc., can be obtained when air is used as the gas mixture instead of pre-packaged and purified sources of oxygen and other gases. Accordingly, in one embodiment the disclosure provides a method for performing the OCM reaction in air by use of one or more of the disclosed catalysts.

In addition to air or O₂ gas, the presently disclosed catalysts and associated methods provide for use of other sources of oxygen in the OCM reaction. In this respect, an alternate source of oxygen such a CO₂, H₂O, SO₂ or SO₃ may be used either in place of, or in addition to, air or oxygen as the oxygen source. Such methods have the potential to increase the efficiency of the OCM reaction, for example by consuming a reaction byproduct (e.g., CO₂ or H₂O) and controlling the OCM exotherm as described below.

As noted above, in the OCM reaction, methane is oxidatively converted to methyl radicals, which are then coupled to form ethane, which is subsequently oxidized to ethylene. In traditional OCM reactions, the oxidation agent for both the methyl radical formation and the ethane oxidation to ethylene is oxygen. In order to minimize full oxidation of methane or ethane to carbon dioxide, i.e., maximize C2+ selectivity, the methane to oxygen ratio is generally kept at 4 (i.e., full conversion of methane into methyl radicals) or above. As a result, the OCM reaction is typically oxygen limited and thus the oxygen concentration in the effluent is zero.

Accordingly, in one embodiment the present disclosure provides a method for increasing the methane conversion and increasing, or in some embodiments, not reducing, the C2+ selectivity in an OCM reaction. The disclosed methods include adding to a traditional OCM catalyst another OCM catalyst that uses an oxygen source other than molecular oxygen. In some embodiments, the alternate oxygen source is CO₂, H₂O, SO₂, SO₃ or combinations thereof. For example in some embodiments, the alternate oxygen source is CO₂. In other embodiments the alternate oxygen source is H₂O.

Because C2+ selectivity is typically between 50% and 80% in the OCM reaction, OCM typically produces significant amounts of CO₂ as a byproduct (CO₂ selectivity can typically range from 20-50%). Additionally, H₂O is produced in copious amounts, regardless of the C2+ selectivity. Therefore both CO₂ and H₂O are attractive oxygen sources for OCM in an O₂ depleted environment. Accordingly, one embodiment of the present disclosure provides a catalyst (and related methods for use thereof) which is catalytic in the OCM reaction and which uses CO₂, H₂O, SO₂, SO₃ or another alternative oxygen source or combinations thereof as a source of oxygen. Other embodiments, provide a catalytic material comprising two or more catalysts, wherein the catalytic material comprises at least one catalyst which is catalytic in the OCM reaction and uses O₂ for at least one oxygen source and at least one catalysts which is catalytic in the OCM reaction and uses at least of CO₂, H₂O, SO₂, SO₃, NO, NO₂, NO₃ or another alternative oxygen source. Methods for performing the OCM reaction with such catalytic materials are also provided. Such catalysts comprise any of the compositions disclosed herein and are effective as catalysts in an OCM reaction using an alternative oxygen source at temperatures of 900° C. or lower, 850° C. or lower, 800° C. or lower, 750° C. or lower, 700° C. or lower or even 650° C. or lower.

Examples of OCM catalysts that use CO₂ or other oxygen sources rather than O₂ include, but are not limited to, catalysts comprising La₂O₃/ZnO, CeO₂/ZnO, CaO/ZnO, CaO/CeO₂, CaO/Cr₂O₃, CaO/MnO₂, SrO/ZnO, SrO/CeO₂, SrO/Cr₂O₃, SrO/MnO₂, SrCO₃/MnO₂, BaO/ZnO, BaO/CeO₂, BaO/Cr₂O₃, BaO/MnO₂, CaO/MnO/CeO₂, Na₂WO₄/Mn/SiO₂, Pr₂O₃, or Tb₂O₃.

Some embodiments provide a method for performing OCM, wherein a mixture of an OCM catalyst which use O₂ as an oxygen source (referred to herein as an O₂-OCM catalyst) and an OCM catalyst which use CO₂ as an oxygen source (referred to herein as a CO₂-OCM catalyst) is employed as the catalytic material, for example in a catalyst bed. Such methods have certain advantages. For example, the CO₂-OCM reaction is endothermic and the O₂-OCM reaction is exothermic, and thus if the right mixture and/or arrangement of CO₂-OCM and O₂-OCM catalysts is used, the methods are particularly useful for controlling the exotherm of the OCM reaction. In some embodiments, the catalyst bed comprises a mixture of O₂-OCM catalyst and CO₂-OCM catalysts. The mixture may be in a ratio of 1:99 to 99:1. The two catalysts work synergistically as the O₂-OCM catalyst supplies the CO₂-OCM catalyst with the necessary carbon dioxide and the endothermic nature of the C₂-OCM reaction serves to control the exotherm of the overall reaction. Alternatively, the CO₂ source may be external to the reaction (e.g., fed in from a CO₂ tank, or other source) and/or the heat required for the CO₂-OCM reaction is supplied from an external source (e.g., heating the reactor).

Since the gas composition will tend to become enriched in CO₂ as it flows through the catalyst bed (i.e., as the OCM reaction proceeds, more CO₂ is produced), some embodiments of the present invention provide an OCM method wherein the catalyst bed comprises a gradient of catalysts which changes from a high concentration of O₂-OCM catalysts at the front of the bed to a high concentration of CO₂-OCM catalysts at the end of the catalyst bed.

The O₂-OCM catalyst and CO₂ OCM catalyst may have the same or different compositions. For example, in some embodiments the O₂-OCM catalyst and CO₂-OCM catalyst have the same composition but different morphologies (e.g., nanowire, bent nanowire, bulk, etc.). In other embodiments the O₂-OCM and the CO₂-OCM catalyst have different compositions.

Furthermore, CO₂-OCM catalysts will typically have higher selectivity, but lower yields than an O₂-OCM catalyst. Accordingly, in one embodiment the methods comprise use of a mixture of an O₂-OCM catalyst and a CO₂-OCM catalyst and performing the reaction in O₂ deprived environment so that the CO₂-OCM reaction is favored and the selectivity is increased. Under appropriate conditions the yield and selectivity of the OCM reaction can thus be optimized.

In some other embodiments, the catalyst bed comprises a mixture of one or more low temperature O₂-OCM catalyst (i.e., a catalyst active at low temperatures, for example less than 700° C.) and one or more high temperature CO₂-OCM catalyst (i.e., a catalyst active at high temperatures, for example 800° C. or higher). Here, the required high temperature for the CO₂-OCM may be provided by the hotspots produced by the O₂-OCM catalyst. In such a scenario, the mixture may be sufficiently coarse such that the hotspots are not being excessively cooled down by excessive dilution effect.

In other embodiments, the catalyst bed comprises alternating layers of O₂-OCM and CO₂-OCM catalysts. The catalyst layer stack may begin with a layer of O₂-OCM catalyst, so that it can supply the next layer (e.g., a CO₂-OCM layer) with the necessary CO₂. The O₂-OCM layer thickness may be optimized to be the smallest at which O2 conversion is 100% and thus the CH₄ conversion of the layer is maximized. The catalyst bed may comprise any number of catalyst layers, for example the overall number of layers may be optimized to maximize the overall CH₄ conversion and C2+ selectivity.

In some embodiments, the catalyst bed comprises alternating layers of low temperature O₂-OCM catalysts and high temperature CO₂-OCM catalysts. Since the CO₂-OCM reaction is endothermic, the layers of CO₂-OCM catalyst may be sufficiently thin such that in can be “warmed up” by the hotspots of the O₂-OCM layers. The endothermic nature of the CO₂-OCM reaction can be advantageous for the overall thermal management of an OCM reactor. In some embodiments, the CO₂-OCM catalyst layers act as “internal” cooling for the O₂-OCM layers, thus simplifying the requirements for the cooling, for example in a tubular reactor. Therefore, an interesting cycle takes place with the endothermic reaction providing the necessary heat for the endothermic reaction and the endothermic reaction providing the necessary cooling for the exothermic reaction.

Accordingly, one embodiment of the present invention is a method for the oxidative coupling of methane, wherein the method comprises conversion of methane to ethane and/or ethylene in the presence of a catalytic material, and wherein the catalytic material comprises a bed of alternating layers of O₂-OCM catalysts and CO₂-OCM catalysts. In other embodiments the bed comprises a mixture (i.e., not alternating layers) of O₂-OCM catalysts and CO₂-OCM catalysts.

In other embodiments, the OCM methods include use of a jacketed reactor with the exothermic O₂-OCM reaction in the core and the endothermic CO₂-OCM reaction in the mantel. In other embodiments, the unused CO₂ can be recycled and reinjected into the reactor, optionally with the recycled CH₄. Additional CO₂ can also be injected to increase the overall methane conversion and help reduce greenhouse gases.

In other embodiments, the reactor comprises alternating stages of O₂-OCM catalyst beds and CO₂-OCM catalyst beds. The CO₂ necessary for the CO₂-OCM stages is provided by the O₂-OCM stage upstream. Additional CO₂ may also be injected. The O₂ necessary for the subsequent O₂-OCM stages is injected downstream from the CO₂-OCM stages. The CO₂-OCM stages may provide the necessary cooling for the O₂-OCM stages. Alternatively, separate cooling may be provided. Likewise, if necessary the inlet gas of the CO₂-OCM stages can be additionally heated, the CO₂-OCM bed can be heated or both.

In related embodiments, the CO₂ naturally occurring in natural gas is not removed prior to performing the OCM, alternatively CO₂ is added to the feed with the recycled methane. Instead the CO₂ containing natural gas is used as a feedstock for CO₂-OCM, thus potentially saving a separation step. The amount of naturally occurring CO₂ in natural gas depends on the well and the methods can be adjusted accordingly depending on the source of the natural gas.

The foregoing methods can be generalized as a method to control the temperature of very exothermic reactions by coupling them with an endothermic reaction that uses the same feedstock (or byproducts of the exothermic reaction) to make the same product (or a related product). This concept can be reversed, i.e., providing heat to an endothermic reaction by coupling it with an exothermic reaction. This will also allow a higher per pass yield in the OCM reactor.

For purpose of simplicity, the above description relating to the use of O₂-OCM and CO₂-OCM catalysts was described in reference to the oxidative coupling of methane (OCM); however, the same concept is applicable to other catalytic reactions including but not limited to: oxidative dehydrogenation (ODH) of alkanes to their corresponding alkenes, selective oxidation of alkanes and alkenes and alkynes, etc. For example, in a related embodiment, a catalyst capable of using an alternative oxygen source (e.g., CO₂, H₂O, SO₂, SO₃ or combinations thereof) to catalyze the oxidative dehydrogenation of ethane is provided. Such catalysts, and uses thereof are described in more detail below.

Furthermore, the above methods are applicable for creating novel catalysts by blending catalysts that use different reactants for the same catalytic reactions, for example different oxidants for an oxidation reaction and at least one oxidant is a byproduct of one of the catalytic reactions. In addition, the methods can also be generalized for internal temperature control of reactors by blending catalysts that catalyze reactions that share the same or similar products but are exothermic and endothermic, respectively. These two concepts can also be coupled together.

2. Oxidative Dehydrogenation

Worldwide demand for alkenes, especially ethylene and propylene, is high. The main sources for alkenes include steam cracking, fluid-catalytic-cracking and catalytic dehydrogenation. The current industrial processes for producing alkenes, including ethylene and propylene, suffer from some of the same disadvantages described above for the OCM reaction. Accordingly, a process for the preparation of alkenes which is more energy efficient and has higher yield, selectivity, and conversion than current processes is needed. The catalysts disclosed herein fulfill this need and provide related advantages.

In one embodiment, the catalysts are useful for the oxidative dehydrogenation (ODH) of hydrocarbons (e.g., alkanes, alkenes, and alkynes). For example, in one embodiment the catalysts are useful in an ODH reaction for the conversion of ethane or propane to ethylene or propylene, respectively. Reaction scheme (9) depicts the oxidative dehydrogenation of hydrocarbons: C_(x)H_(y)+½O₂→C_(x)H_(y-2)+H₂O  (9)

Representative catalysts useful for the ODH reaction include, but are not limited to any of the catalysts disclosed herein.

As noted above, improvements to the yield, selectivity, and/or conversion in the ODH reaction employing bulk catalysts are needed. Accordingly, in one embodiment, the catalysts possess a catalytic activity in the ODH reaction such that the yield, selectivity, and/or conversion is better than when the ODH reaction is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the conversion of hydrocarbon to alkene in the ODH reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the conversion of alkane to alkene compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of alkane to alkene in an ODH reaction catalyzed by the catalyst is greater than 10%, greater than 15%, greater than 20%, greater than 25%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the yield of alkene in an ODH reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the yield of alkene compared to the same reaction under the same conditions but performed with a corresponding catalyst. In some embodiments the yield of alkene in an ODH reaction catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity in the ODH reaction such that the nanowire has the same catalytic activity, but at a lower temperature, compared to a corresponding catalyst. In some embodiments the catalytic activity of the catalysts in the ODH reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 20° C. less. In some embodiments the catalytic activity of the catalysts in the ODH reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 50° C. less. In some embodiments the catalytic activity of the catalysts in the ODH reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 100° C. less. In some embodiments the catalytic activity of the catalysts in the ODH reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 200° C. less.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the selectivity for alkenes in an ODH reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the selectivity for alkenes compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the selectivity for alkenes in an ODH reaction catalyzed by the catalyst is greater than 50%, greater than 60%, greater than 70%, greater than 75%, greater than 80%, greater than 85%, greater than 90%, or greater than 95%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the selectivity for CO or CO₂ in an ODH reaction is less than at least 0.9 times, 0.8 times, 0.5 times, 0.2 times, or 0.1 times the selectivity for CO or CO₂ compared to the same reaction under the same conditions but performed with a corresponding catalyst.

In one embodiment, the catalysts disclosed herein enable efficient conversion of alkane to alkene in the ODH reaction at temperatures less than when a corresponding catalyst is used. For example, in one embodiment, the catalysts disclosed herein enable efficient conversion (i.e., high yield, conversion, and/or selectivity) of hydrocarbon to alkene at temperatures of less than 800° C., less than 700° C., less than 600° C., less than 500° C., less than 400° C., or less than 300° C.

The stability of the catalysts is defined as the length of time a catalyst will maintain its catalytic performance without a significant decrease in performance (e.g., a decrease >20%, >15%, >10%, >5%, or greater than 1% in ODH activity or alkene selectivity, etc.). In some embodiments, the catalysts have stability under conditions required for the ODH reaction of >1 hr, >5 hrs, >10 hrs, >20 hrs, >50 hrs, >80 hrs, >90 hrs, >100 hrs, >150 hrs, >200 hrs, >250 hrs, >300 hrs, >350 hrs, >400 hrs, >450 hrs, >500 hrs, >550 hrs, >600 hrs, >650 hrs, >700 hrs, >750 hrs, >800 hrs, >850 hrs, >900 hrs, >950 hrs, >1,000 hrs, >2,000 hrs, >3,000 hrs, >4,000 hrs, >5,000 hrs, >6,000 hrs, >7,000 hrs, >8,000 hrs, >9,000 hrs, >10,000 hrs, >11,000 hrs, >12,000 hrs, >13,000 hrs, >14,000 hrs, >15,000 hrs, >16,000 hrs, >17,000 hrs, >18,000 hrs, >19,000 hrs, >20,000 hrs, >1 yrs, >2 yrs, >3 yrs, >4 yrs or >5 yrs.

One embodiment of the present disclosure is directed to a catalyst capable of using an alternative oxygen source (e.g., CO₂, H₂O, SO₂, SO₃ or combinations thereof) to catalyze the oxidative dehydrogenation of ethane. For example, the ODH reaction may proceed according to the following reaction (10): CO₂+C_(x)H_(y)→C_(x)H_(y-2)+CO+H₂O  (10) wherein x is an integer and Y is 2x+2. Compositions useful in this regard include Fe₂O₃, Cr₂O₃, MnO₂, Ga₂O₃, Cr/SiO₂, Cr/SO₄—SiO₂, Cr—K/SO₄—SiO₂, Na₂WO₄—Mn/SiO₂, Cr-HZSM-5, Cr/Si-MCM-41 (Cr-HZSM-5 and Cr/Si-MCM-41 refer to known zeolites) and MoC/SiO₂. In some embodiments, any of the foregoing catalyst compositions may be supported on SiO₂, ZrO₂, Al₂O₃, TiO₂ or combinations thereof.

The catalysts having ODH activity with alternative oxygen sources (e.g., CO₂, referred to herein as a CO₂-ODH catalyst) have a number of advantages. For example, in some embodiments a method for converting methane to ethylene comprises use of an O₂-OCM catalyst in the presence of a CO₂-ODH catalyst is provided. Catalytic materials comprising at least one O₂-OCM catalyst and at least one CO₂-ODH catalyst are also provided in some embodiments. This combination of catalysts results in a higher yield of ethylene (and/or ratio of ethylene to ethane) since the CO₂ produced by the OCM reaction is consumed and used to convert ethane to ethylene.

In one embodiment, a method for preparation of ethylene comprises converting methane to ethylene in the presence of two or more catalysts, wherein at least one catalyst is an O₂-OCM catalyst and at least one catalyst is a CO₂-ODH catalyst. Such methods have certain advantages. For example, the CO2-ODH reaction is endothermic and the O₂-OCM reaction is exothermic, and thus if the right mixture and/or arrangement of CO₂-ODH and O₂-OCM catalysts is used, the methods are particularly useful for controlling the exotherm of the OCM reaction. In some embodiments, the catalyst bed comprises a mixture of O₂-OCM catalyst and CO2-ODH catalysts. The mixture may be in a ratio of 1:99 to 99:1. The two catalysts work synergistically as the O₂-OCM catalyst supplies the CO₂-ODH catalyst with the necessary carbon dioxide and the endothermic nature of the C₂-OCM reaction serves to control the exotherm of the overall reaction.

Since the gas composition will tend to become enriched in CO₂ as it flows through the catalyst bed (i.e., as the OCM reaction proceeds, more CO₂ is produced), some embodiments of the present invention provide an OCM method wherein the catalyst bed comprises a gradient of catalysts which changes from a high concentration of O₂-OCM catalysts at the front of the bed to a high concentration of CO₂-ODH catalysts at the end of the catalyst bed.

The O₂-ODH catalyst and CO₂-ODH catalyst may have the same or different compositions. For example, in some embodiments the O₂-ODH catalyst and CO₂-ODH catalyst have the same composition but different morphologies (e.g., nanowire, bent nanowire, bulk, etc.). In other embodiments the O₂-ODH and the CO₂-ODH catalyst have different compositions.

In other embodiments, the catalyst bed comprises alternating layers of O₂-OCM and CO₂-ODH catalysts. The catalyst layer stack may begin with a layer of O₂-OCM catalyst, so that it can supply the next layer (e.g., a CO2-ODH layer) with the necessary CO₂. The O₂-OCM layer thickness may be optimized to be the smallest at which O2 conversion is 100% and thus the CH₄ conversion of the layer is maximized. The catalyst bed may comprise any number of catalyst layers, for example the overall number of layers may be optimized to maximize the overall CH₄ conversion and C2+ selectivity.

In some embodiments, the catalyst bed comprises alternating layers of low temperature O₂-OCM catalysts and high temperature CO₂-ODH catalysts. Since the CO₂-ODH reaction is endothermic, the layers of CO₂-ODH catalyst may be sufficiently thin such that in can be “warmed up” by the hotspots of the O₂-OCM layers. The endothermic nature of the CO₂-ODH reaction can be advantageous for the overall thermal management of an OCM reactor. In some embodiments, the CO₂-ODH catalyst layers act as “internal” cooling for the O₂-OCM layers, thus simplifying the requirements for the cooling, for example in a tubular reactor. Therefore, an interesting cycle takes place with the endothermic reaction providing the necessary heat for the endothermic reaction and the endothermic reaction providing the necessary cooling for the exothermic reaction.

Accordingly, one embodiment of the present invention is a method for the oxidative coupling of methane, wherein the method comprises conversion of methane to ethane and/or ethylene in the presence of a catalytic material, and wherein the catalytic material comprises a bed of alternating layers of O₂-OCM catalysts and CO₂-ODH catalysts. In other embodiments the bed comprises a mixture (i.e., not alternating layers) of O₂-OCM catalysts and CO₂-ODH catalysts. Such methods increase the ethylene yield and/or ratio of ethylene to ethane compared to other known methods.

In other embodiments, the OCM methods include use of a jacketed reactor with the exothermic O₂-OCM reaction in the core and the endothermic CO₂-ODH reaction in the mantel. In other embodiments, the unused CO₂ can be recycled and reinjected into the reactor, optionally with the recycled CH₄. Additional CO₂ can also be injected to increase the overall methane conversion and help reduce greenhouse gases.

In other embodiments, the reactor comprises alternating stages of O₂-OCM catalyst beds and CO₂-ODH catalyst beds. The CO₂ necessary for the CO₂-ODH stages is provided by the O₂-OCM stage upstream. Additional CO₂ may also be injected. The O₂ necessary for the subsequent O2-OCM stages is injected downstream from the CO₂-ODH stages. The CO₂-ODH stages may provide the necessary cooling for the O₂-OCM stages. Alternatively, separate cooling may be provided. Likewise, if necessary the inlet gas of the CO₂-ODH stages can be additionally heated, the CO₂-ODH bed can be heated or both.

In related embodiments, the CO₂ naturally occurring in natural gas is not removed prior to performing the OCM, alternatively CO₂ is added to the feed with the recycled methane. Instead the CO₂ containing natural gas is used as a feedstock for CO₂-ODH, thus potentially saving a separation step. The amount of naturally occurring CO₂ in natural gas depends on the well and the methods can be adjusted accordingly depending on the source of the natural gas.

3. Carbon Dioxide Reforming of Methane

Carbon dioxide reforming (CDR) of methane is an attractive process for converting CO₂ in process streams or naturally occurring sources into the valuable chemical product, syngas (a mixture of hydrogen and carbon monoxide). Syngas can then be manufactured into a wide range of hydrocarbon products through processes such as the Fischer-Tropsch synthesis (discussed below) to form liquid fuels including methanol, ethanol, diesel, and gasoline. The result is a powerful technique to not only remove CO₂ emissions but also create a new alternative source for fuels that are not derived from petroleum crude oil. The CDR reaction with methane is exemplified in reaction scheme (11). CO₂+CH₄→2CO+2H₂  (11)

Unfortunately, no established industrial technology for CDR exists today in spite of its tremendous potential value. While not wishing to be bound by theory, it is thought that the primary problem with CDR is due to side-reactions from catalyst deactivation induced by carbon deposition via the Boudouard reaction (reaction scheme (12)) and/or methane cracking (reaction scheme (13)) resulting from the high temperature reaction conditions. The occurrence of the coking effect is intimately related to the complex reaction mechanism, and the associated reaction kinetics of the catalysts employed in the reaction. 2CO→C+CO₂  (12) CH₄→C+2H₂  (13)

While not wishing to be bound by theory, the CDR reaction is thought to proceed through a multistep surface reaction mechanism. FIG. 3 schematically depicts a CDR reaction 700, in which activation and dissociation of CH₄ occurs on the metal catalyst surface 710 to form intermediate “M-C”. At the same time, absorption and activation of CO₂ takes place at the oxide support surface 720 to provide intermediate “S—CO₂”, since the carbon in a CO₂ molecule as a Lewis acid tends to react with the Lewis base center of an oxide. The final step is the reaction between the M-C species and the activated S—CO₂ to form CO.

In one embodiment, the catalysts disclosed herein are useful as catalysts for the carbon dioxide reforming of methane. For example, in one embodiment the catalysts are useful as catalysts in a CDR reaction for the production of syn gas.

Improvements to the yield, selectivity, and/or conversion in the CDR reaction employing known catalysts are needed. Accordingly, in one embodiment, the catalysts possess a catalytic activity in the CDR reaction such that the yield, selectivity, and/or conversion is better than when the CDR reaction is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the conversion of CO₂ to CO in the CDR reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the conversion of CO₂ to CO compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of CO₂ to CO in a CDR reaction catalyzed by the catalyst is greater than 10%, greater than 15%, greater than 20%, greater than 25%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the yield of CO in a CDR reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the yield of CO compared to the same reaction under the same conditions but performed with a corresponding catalyst. In some embodiments the yield of CO in a CDR reaction catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the selectivity for CO in a CDR reaction is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the selectivity for CO compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the selectivity for CO in a CDR reaction catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 40%, greater than 50%, greater than 65%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity in a CDR reaction such that the catalyst has the same or better catalytic activity, but at a lower temperature, compared to a corresponding. In some embodiments the catalytic activity of the catalysts in a CDR reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 20° C. less. In some embodiments the catalytic activity of the catalysts in a CDR reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 50° C. less. In some embodiments the catalytic activity of the catalysts in a CDR reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 100° C. less. In some embodiments the catalytic activity of the catalysts in a CDR reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 200° C. less.

In one embodiment, the catalysts disclosed herein enable efficient conversion of CO₂ to CO in the CDR reaction at temperatures less than when a corresponding catalyst is used. For example, in one embodiment, the catalysts enable efficient conversion (i.e., high yield, conversion, and/or selectivity) of CO₂ to CO at temperatures of less than 900° C., less than 800° C., less than 700° C., less than 600° C., or less than 500° C.

4. Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis (FTS) is a valuable process for converting synthesis gas (i.e., CO and H₂) into valuable hydrocarbon fuels, for example, light alkenes, gasoline, diesel fuel, etc. FTS has the potential to reduce the current reliance on the petroleum reserve and take advantage of the abundance of coal and natural gas reserves. Current FTS processes suffer from poor yield, selectivity, conversion, catalyst deactivation, poor thermal efficiency and other related disadvantages. Production of alkanes via FTS is shown in reaction scheme (14), wherein n is an integer. CO+2H₂→(1/n)(C_(n)H_(2n))+H₂O  (14)

In one embodiment, the catalysts are useful as catalysts in FTS processes. For example, in one embodiment the catalysts are useful as catalysts in a FTS process for the production of alkanes.

Improvements to the yield, selectivity, and/or conversion in FTS processes employing bulk catalysts are needed. Accordingly, in one embodiment, the catalysts possess a catalytic activity in an FTS process such that the yield, selectivity, and/or conversion is better than when the FTS process is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the conversion of CO to alkane in an FTS process is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the conversion of CO to alkane compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of CO to alkane in an FTS process catalyzed by the catalyst is greater than 10%, greater than 15%, greater than 20%, greater than 25%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity in an FTS process such that the catalyst has the same or better catalytic activity, but at a lower temperature, compared a corresponding catalyst. In some embodiments, the catalytic activity of the catalysts in an FTS process is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 20° C. less. In some embodiments the catalytic activity of the catalysts in an FTS process is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 50° C. less. In some embodiments the catalytic activity of the catalysts in an FTS process is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 100° C. less. In some embodiments the catalytic activity of the catalysts in an FTS process is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 200° C. less.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the yield of alkane in a FTS process is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the yield of alkane compared to the same reaction under the same conditions but performed with a corresponding catalyst. In some embodiments the yield of alkane in an FTS process catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 40%, greater than 50%, greater than 65%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the selectivity for alkanes in an FTS process is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the selectivity for alkanes compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the selectivity for alkanes in an FTS process catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In one embodiment, the catalysts disclosed herein enable efficient conversion of CO to alkanes in a CDR process at temperatures less than when a corresponding catalyst is used. For example, in one embodiment, the catalysts enable efficient conversion (i.e., high yield, conversion, and/or selectivity) of CO to alkanes at temperatures of less than 400° C., less than 300° C., less than 250° C., less than 200° C., less the 150° C., less than 100° C. or less than 50° C.

5. Oxidation of CO

Carbon monoxide (CO) is a toxic gas and can convert hemoglobin to carboxyhemoglobin resulting in asphyxiation. Dangerous levels of CO can be reduced by oxidation of CO to CO₂ as shown in reaction scheme 15: CO+½O₂→CO₂  (15)

Catalysts for the conversion of CO into CO₂ have been developed but improvements to the known catalysts are needed. Accordingly in one embodiment, the present disclosure provides catalysts useful as catalysts for the oxidation of CO to CO₂.

In one embodiment, the catalysts possess a catalytic activity in a process for the conversion of CO into CO₂ such that the yield, selectivity, and/or conversion is better than when the oxidation of CO into CO₂ is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the conversion of CO to CO₂ is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the conversion of CO to CO₂ compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the conversion of CO to CO₂ catalyzed by the catalyst is greater than 10%, greater than 15%, greater than 20%, greater than 25%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the yield of CO₂ from the oxidation of CO is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the yield of CO₂ compared to the same reaction under the same conditions but performed with a corresponding catalyst. In some embodiments the yield of CO₂ from the oxidation of CO catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity in an oxidation of CO reaction such that the catalyst has the same or better catalytic activity, but at a lower temperature, compared to a corresponding catalyst. In some embodiments the catalytic activity of the catalysts in an oxidation of CO reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 20° C. less. In some embodiments the catalytic activity of the catalysts in an oxidation of CO reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 50° C. less. In some embodiments the catalytic activity of the catalysts in an oxidation of CO reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 100° C. less. In some embodiments the catalytic activity of the catalysts in an oxidation of CO reaction is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 200° C. less.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the selectivity for CO₂ in the oxidation of CO is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the selectivity for CO₂ compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the selectivity for CO₂ in the oxidation of CO catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 40%, greater than 50%, greater than 65%, greater than 75%, or greater than 90%.

In one embodiment, the catalysts disclosed herein enable efficient conversion of CO to CO₂ at temperatures less than when a corresponding catalyst is used as a catalyst. For example, in one embodiment, the catalysts enable efficient conversion (i.e., high yield, conversion, and/or selectivity) of CO to CO₂ at temperatures of less than 500° C., less than 400° C., less than 300° C., less than 200° C., less than 100° C., less than 50° C. or less than 20° C.

Although various reactions have been described in detail, the disclosed catalysts are useful as catalysts in a variety of other reactions. In general, the disclosed catalysts find utility in any reaction utilizing a heterogeneous catalyst and have a catalytic activity such that the yield, conversion, and/or selectivity in reaction catalyzed by the catalysts is better than the yield, conversion and/or selectivity in the same reaction catalyzed by a corresponding catalyst.

6. Combustion of Hydrocarbons

In another embodiment, the present disclosure provides a catalyst having catalytic activity in a reaction for the catalyzed combustion of hydrocarbons. Such catalytic reactions find utility in catalytic converters for automobiles, for example by removal of unburned hydrocarbons in the exhaust by catalytic combustion or oxidation of soot captured on catalyzed particle filters resulting in reduction on diesel emissions from the engine. When running “cold”, the exhausts temperature of a diesel engine is quite low, thus a low temperature catalyst, such as the disclosed catalysts, is needed to efficiently eliminate all unburned hydrocarbons. In addition, in case of soot removal on catalyzed particulate filters, intimate contact between the soot and the catalyst is require; the open mesh morphology of catalyst coating is advantageous to promote such intimate contact between soot and oxidation catalyst.

In contrast to a corresponding catalyst, Applicants have found that certain catalysts, for example the exemplary catalysts disclosed herein, possess a catalytic activity (for example because of their morphology) in the combustion of hydrocarbons or soot such that the yield, selectivity, and/or conversion is better than when the combustion of hydrocarbons is catalyzed by a corresponding catalyst. In one embodiment, the disclosure provides a catalyst having a catalytic activity such that the combustion of hydrocarbons is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the combustion of hydrocarbons compared to the same reaction under the same conditions but performed with a corresponding catalyst. In other embodiments, the total combustion of hydrocarbons catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

In another embodiment, the disclosure provides a catalyst having a catalytic activity such that the yield of combusted hydrocarbon products is greater than at least 1.1 times, 1.25 times, 1.50 times, 2.0 times, 3.0 times, or 4.0 times the yield of combusted hydrocarbon products compared to the same reaction under the same conditions but performed with a corresponding catalyst. In some embodiments the yield of combusted hydrocarbon products in a reaction catalyzed by the catalyst is greater than 10%, greater than 20%, greater than 30%, greater than 50%, greater than 75%, or greater than 90%.

The stability of the catalysts is defined as the length of time a catalyst will maintain its catalytic performance without a significant decrease in performance (e.g., a decrease >20%, >15%, >10%, >5%, or greater than 1% in hydrocarbon or soot combustion activity). In some embodiments, the catalysts have stability under conditions required for the hydrocarbon combustion reaction of >1 hr, >5 hrs, >10 hrs, >20 hrs, >50 hrs, >80 hrs, >90 hrs, >100 hrs, >150 hrs, >200 hrs, >250 hrs, >300 hrs, >350 hrs, >400 hrs, >450 hrs, >500 hrs, >550 hrs, >600 hrs, >650 hrs, >700 hrs, >750 hrs, >800 hrs, >850 hrs, >900 hrs, >950 hrs, >1,000 hrs, >2,000 hrs, >3,000 hrs, >4,000 hrs, >5,000 hrs, >6,000 hrs, >7,000 hrs, >8,000 hrs, >9,000 hrs, >10,000 hrs, >11,000 hrs, >12,000 hrs, >13,000 hrs, >14,000 hrs, >15,000 hrs, >16,000 hrs, >17,000 hrs, >18,000 hrs, >19,000 hrs, >20,000 hrs, >1 yrs, >2 yrs, >3 yrs, >4 yrs or >5 yrs.

In another embodiment, the disclosure provides a catalyst having a catalytic activity in the combustion of hydrocarbons such that the catalyst has the same or better catalytic activity, but at a lower temperature, compared to a corresponding catalyst. In some embodiments the catalytic activity of the catalysts in the combustion of hydrocarbons is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 20° C. less. In some embodiments the catalytic activity of the catalysts in the combustion of hydrocarbons is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 50° C. less. In some embodiments the catalytic activity of the catalysts in the combustion of hydrocarbons is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 100° C. less. In some embodiments the catalytic activity of the catalysts in the combustion of hydrocarbons is the same or better than the catalytic activity of a corresponding catalyst, but at a temperature of at least 200° C. less.

7. Evaluation of Catalytic Properties

To evaluate the catalytic properties of the catalysts in a given reaction, for example those reactions discussed above, various methods can be employed to collect and process data including measurements of the kinetics and amounts of reactants consumed and the products formed. In addition to allowing for the evaluation of the catalytic performances, the data can also aid in designing large scale reactors, experimentally validating models and optimizing the catalytic process.

One exemplary methodology for collecting and processing data is depicted in FIG. 4. Three main steps are involved. The first step (block 750) comprises the selection of a reaction and catalyst. This influences the choice of reactor and how it is operated, including batch, flow, etc. (block 754). Thereafter, the data of the reaction are compiled and analyzed (block 760) to provide insights to the mechanism, rates and process optimization of the catalytic reaction. In addition, the data provide useful feedbacks for further design modifications of the reaction conditions. Additional methods for evaluating catalytic performance in the laboratory and industrial settings are described in, for example, Bartholomew, C. H. et al. Fundamentals of Industrial Catalytic Processes, Wiley-AIChE; 2Ed (1998).

As an example, in a laboratory setting, an Altamira Benchcat 200 can be employed using a 4 mm ID diameter quartz tube with a 0.5 mm ID capillary downstream. Quartz tubes with 2 mm or 6 mm ID can also be used. Catalysts are tested in a number of different dilutions and amounts. In some embodiments, the range of testing is between 10 and 300 mg. In some embodiments, the catalysts are diluted with a non-reactive diluent. This diluent can be quartz (SiO₂) or other inorganic materials, which are known to be inert in the reaction condition. The purpose of the diluent is to minimize hot spots and provide an appropriate loading into the reactor. In addition, the catalyst can be blended with less catalytically active components as described in more detail above.

In a typical procedure, 50 mg is the total charge of catalyst, optionally including diluent. On either side of the catalysts a small plug of glass wool is loaded to keep the catalysts in place. A thermocouple is placed on the inlet side of the catalyst bed into the glass wool to get the temperature in the reaction zone. Another thermocouple can be placed on the downstream end of the catalyst bed into the catalyst bed itself to measure the exotherms, if any.

When blending the catalyst with diluent, the following exemplary procedure may be used: x (usually 10-50) mg of the catalyst (either bulk or nanowire catalyst) is blended with (100−x) mg of diluent. Thereafter, about 2 ml of ethanol or water is added to form a slurry mixture, which is then sonicated for about 10 minutes. The slurry is then dried in an oven at about 100-140° C. for 2 hours to remove solvent. The resulting solid mixture is then scraped out and loaded into the reactor between the plugs of quartz wool.

Once loaded into the reactor, the reactor is inserted into the Altamira instrument and furnace and then a temperature and flow program is started. In some embodiment, the total flow is 50 to 100 sccm of gases but this can be varied and programmed with time. In one embodiment, the temperatures range from 400° C. to 900° C. The reactant gases comprise air or oxygen (diluted with nitrogen or argon) and methane in the case of the OCM reaction and gas mixtures comprising ethane and/or propane with oxygen for oxidative dehydrogenation (ODH) reactions. Other gas mixtures can be used for other reactions.

The primary analysis of these oxidation catalysis runs is the Gas Chromatography (GC) analysis of the feed and effluent gases. From these analyses, the conversion of the oxygen and alkane feed gases can easily be attained and estimates of yields and selectivities of the products and by-products can be determined.

The GC method developed for these experiments employs up to 4 columns and up to 2 detectors and a complex valve switching system to optimize the analysis. Specifically, a flame ionization detector (FID) is used for the analysis of the hydrocarbons only. It is a highly sensitive detector that produces accurate and repeatable analysis of methane, ethane, ethylene, propane, propylene and all other simple alkanes and alkenes up to five carbons in length and down to ppm levels.

There can be two columns in series to perform this analysis, the first is a stripper column (alumina) which traps polar materials (including the water byproduct and any oxygenates generated) until back-flushed later in the cycle. The second column associated with the FID is a capillary alumina column known as a PLOT column which performs the actual separation of the light hydrocarbons. The water and oxygenates are not analyzed in this method.

For the analysis of the light non-hydrocarbon gases, one or more Thermal Conductivity Detectors (TCD) may be employed which can also employ two columns to accomplish its analysis. The target molecules for this analysis are CO₂, ethylene, ethane, hydrogen, oxygen, nitrogen, methane and CO. The two columns used here are a porous polymer column known as the Hayes Sep N which performs some of the separation for the CO₂, ethylene and ethane. The second column is a molecular sieve column which uses size differentiation to perform the separation. It is responsible for the separation of H₂, O₂, N₂, methane and CO.

There is a sophisticated and timing sensitive switching between these two columns in the method. In the first 2 minutes or so, the two columns are operating in series but at about 2 minutes, the molecular sieve column is by-passed and the separation of the first 3 components is completed. At about 5-7 minutes, the columns are then placed back in series and the light gases come off of the sieve according to their molecular size.

The end result is an accurate analysis of all of the aforementioned components from these fixed-beds, gas phase reactions. Analysis of other reactions and gases not specifically described above can be performed in a similar manner known to those of skill in the art.

8. Downstream Products

As noted above, the catalysts disclosed herein are useful in reactions for the preparation of a number of valuable hydrocarbon compounds. For example, in one embodiment the catalysts are useful for the preparation of ethylene from methane via the OCM reaction. In another embodiment, the catalysts are useful for the preparation of ethylene or propylene via oxidative dehydrogenation of ethane or propane respectively. Ethylene and propylene are valuable compounds which can be converted into a variety of consumer products. For example, as shown in FIG. 5, ethylene can be converted into many various compounds including low density polyethylene, high density polyethylene, ethylene dichloride, ethylene oxide, ethylbenzene, linear alcohols, vinyl acetate, alkanes, alpha olefins, various hydrocarbon-based fuels, ethanol and the like. These compounds can then be further processed using methods well known to one of ordinary skill in the art to obtain other valuable chemicals and consumer products (e.g., the downstream products shown in FIG. 5). Propylene can be analogously converted into various compounds and consumer goods including polypropylenes, propylene oxides, propanol, and the like.

Accordingly, in one embodiment the invention is directed to a method for the preparation of C2 hydrocarbons via the OCM reaction, the method comprises contacting a catalyst as described herein with a gas comprising methane. In some embodiments the C2 hydrocarbons are selected from ethane and ethylene. In other embodiments the disclosure provides a method of preparing downstream products of ethylene. The method comprises converting ethylene into a downstream product of ethylene, wherein the ethylene has been prepared via a catalytic reaction employing a catalyst disclosed herein (e.g., OCM). In some embodiments, the downstream product of ethylene is low density polyethylene, high density polyethylene, ethylene dichloride, ethylene oxide, ethylbenzene, ethanol or vinyl acetate from ethylene, wherein the ethylene has been prepared as described above. In other embodiments, the downstream product of ethylene is natural gasoline. In still other embodiments, the downstream product of ethylene comprises 1-hexene, 1-octene, hexane, octane, benzene, toluene, xylene or combinations thereof.

In another embodiment, a process for the preparation of ethylene from methane comprising contacting a mixture comprising oxygen and methane at a temperature below 900° C., below 850° C., below 800° C., below 750° C., below 700° C. or below 650° C. with a catalyst as disclosed herein is provided.

In another embodiment, the disclosure provides a method of preparing a product comprising low density polyethylene, high density polyethylene, ethylene dichloride, ethylene oxide, ethylbenzene, ethanol or vinyl acetate, alkenes, alkanes, aromatics, alcohols, or mixtures thereof. The method comprises converting ethylene into low density polyethylene, high density polyethylene, ethylene dichloride, ethylene oxide, ethylbenzene, ethanol or vinyl acetate, wherein the ethylene has been prepared via a catalytic reaction employing a catalyst for example any of the exemplary catalysts disclosed herein.

In more specific embodiments of any of the above methods, the ethylene is produced via an OCM or ODH reaction or combinations thereof.

In one particular embodiment, the disclosure provides a method of preparing a downstream product of ethylene and/or ethane, wherein the downstream product is a hydrocarbon fuel. For example, the downstream product of ethylene may be a hydrocarbon fuel such as natural gasoline or a C₄-C₁₄ hydrocarbon, including alkanes, alkenes and aromatics. Some specific examples include 1-butene, 1-hexene, 1-octene, hexane, octane, benzene, toluene, xylenes and the like. The method comprises converting methane into ethylene, ethane or combinations thereof by use of a catalyst, for example any of the catalysts disclosed herein, and further oligomerizing the ethylene and/or ethane to prepare a downstream product of ethylene and/or ethane. For example, the methane may be converted to ethylene, ethane or combinations thereof via the OCM reaction as discussed above.

As depicted in FIG. 6, the method begins with charging methane (e.g., as a component in natural gas) into an OCM reactor. The OCM reaction may then be performed utilizing a catalyst under any variety of conditions. Water and CO₂ are optionally removed from the effluent and unreacted methane is recirculated to the OCM reactor.

Ethylene is recovered and charged to an oligomerization reactor. Optionally the ethylene stream may contain CO₂, H₂O, N₂, ethane, C3's and/or higher hydrocarbons. Oligomerization to higher hydrocarbons (e.g., C₄-C₁₄) then proceeds under any number of conditions known to those of skill in the art. For example oligomerization may be effected by use of any number of catalysts known to those skilled in the art. Examples of such catalysts include catalytic zeolites, crystalline borosilicate molecular sieves, homogeneous metal halide catalysts, Cr catalysts with pyrrole ligands or other catalysts. Exemplary methods for the conversion of ethylene into higher hydrocarbon products are disclosed in the following references: Catalysis Science & Technology (2011), 1(1), 69-75; Coordination Chemistry Reviews (2011), 255(7-8), 861-880; Eur. Pat. Appl. (2011), EP 2287142 A1 20110223; Organometallics (2011), 30(5), 935-941; Designed Monomers and Polymers (2011), 14(1), 1-23; Journal of Organometallic Chemistry 689 (2004) 3641-3668; Chemistry—A European Journal (2010), 16(26), 7670-7676; Acc. Chem. Res. 2005, 38, 784-793; Journal of Organometallic Chemistry, 695 (10-11): 1541-1549 May 15, 2010; Catalysis Today Volume 6, Issue 3, January 1990, Pages 329-349; U.S. Pat. Nos. 5,968,866; 6,800,702; 6,521,806; 7,829,749; 7,867,938; 7,910,670; 7,414,006 and Chem. Commun., 2002, 858-859, each of which are hereby incorporated in their entirety by reference.

In certain embodiments, the exemplary OCM and oligomerization modules depicted in FIG. 6 may be adapted to be at the site of natural gas production, for example a natural gas field. Thus the natural gas can be efficiently converted to more valuable and readily transportable hydrocarbon commodities without the need for transport of the natural gas to a processing facility.

Referring to FIG. 6, “natural gasoline” refers to a mixture of oligomerized ethylene products. In this regard, natural gasoline comprises hydrocarbons containing 5 or more carbon atoms. Exemplary components of natural gasoline include linear, branched or cyclic alkanes, alkenes and alkynes, as well as aromatic hydrocarbons. For example, in some embodiments the natural gasoline comprises 1-pentene, 1-hexene, cyclohexene, 1-octene, benzene, toluene, dimethyl benzene, xylenes, napthalene, or other oligomerized ethylene products or combinations thereof. In some embodiments, natural gasoline may also include C3 and C4 hydrocarbons dissolved within the liquid natural gasoline. This mixture finds particular utility in any number of industrial applications, for example natural gasoline is used as feedstock in oil refineries, as fuel blend stock by operators of fuel terminals, as diluents for heavy oils in oil pipelines and other applications. Other uses for natural gasoline are well-known to those of skill in the art.

The following examples are provided for purposes of illustration, not limitation.

EXAMPLES Example 1 Preparation of Exemplary Bulk Catalysts

Equimolar aqueous solutions of strontium nitrate, neodymium nitrate, and erbium nitrate are prepared. Aliquots of each solution are mixed together to prepare a desired formulation of Er_(x)Nd_(y)Sr_(z) where x, y, z represent mole fractions of total metal content in moles. Representative examples of formulations are: Er₅₀Nd₃₀Sr₂₀, Er₅₂Nd₄₅Sr₀₅, Er₇₅Nd₂₂Sr₀₃, and the like. A solution of citric acid is added to the metal salt mixture so that citric acid mole/metal mole ratio is 3:1. Ethylene glycol is then added to the citric acid/metal salt solution so that the ethylene glycol/citric acid mole ratio is 1:1. The solution is stirred at room temperature for 1 h. The solution is then placed in a 130° C. oven for 15 h to remove water and to promote resin formation. After 15 h, a hard dark resin is observed. The resin is placed in a furnace and heated to 500° C. for 8 h. The remaining material is heated to 650° C. for 2 h to yield the desired product.

Catalysts comprising support materials can also be prepared by coprecipitation according to the above method. For example, rare earth oxides on MgO, CaO or AlPO₄ supports are prepared in an analogous manner. Specific examples include, Er/Nd/Sr/CaO (i.e., a catalyst comprising Er, Nd, and Sr on a CaO support).

Other bulk catalysts are prepared according to an analogous procedure.

Example 2 Preparation of Exemplary Doped Catalyst

3.0 g of Er₂O3 bulk from Alfa Chemicals is slurried in a solution formed by dissolving 0.378 g of Sr(NO₃)₂ in about 20 ml of DI water. The slurry is stirred at room temperature for about 30 minutes to ensure that the Sr(NO₃)₂ dissolves. The slurry is then moved to an evaporating dish and placed into an oven at 100-140° C. for 2-3 hours to ensure dryness. The solids are then calcined in a furnace by ramping up to 350° C. at 5° C./min and holding for 2 hours and then ramping again at the same rate to 700° C. and holding for 4 hours. It is then cooled to room temperature, ground and sieved to a particle size range of 180 μm to 250 μm.

Catalysts comprising different dopants are prepared according to the above general procedure.

Example 3 Preparation of Exemplary Nanowires Catalysts

Phage is prepared as described in U.S. Pub. No. 2012/004124623, the full disclosure of which is incorporated herein by reference. 23 ml of 2.5 e12 pfu solution of phages is mixed in a 40 ml glass bottle with 0.046 ml of 0.1 M ErCl₃ aqueous solution and left incubating for 16 h. After this incubation period, a slow multistep addition is conducted with 1.15 ml of 0.05 M ErCl₃ solution and 1.84 ml of 0.3 M NH₄OH. This addition is conducted in six hours and twenty steps. The reaction mixture is left stirring another 2 h at room temperature. After that time the suspension is centrifuged in order to separate the solid phase from the liquid phase. The precipitated material is then resuspended in 5 ml of water and centrifuged in order to further remove un-reacted species. A final wash is conducted with 2 ml ethanol. The gel-like product remaining is then dried for 30 minutes at 110° C. in a vacuum oven.

The dried product is then calcined in a muffle furnace using a step recipe (load in the furnace at room temperature, ramp to 200° C. with 3° C./min rate, dwell for 120 min, ramp to 400° C. with 3° C./min rate, dwell for 120 min, cool to room temperature). The calcined product is then ground to a fine powder.

5 mg of the calcined product are impregnated with 0.015 ml Sr(NO₃)₂ 0.1M aqueous solution. Powder and solution is mixed on hot plate at 90° C. until a paste is formed. The paste is then dried for 1 h at 120° C. in a vacuum oven and finally calcined in a muffle oven in air. (load in the furnace at room temperature, ramp to 200° C. with 3° C./min rate, dwell for 120 min, ramp to 400° C. with 3° C./min rate, dwell for 120 min, ramp to 500° C. with 3° C./min rate, dwell for 120 min, cool to room temperature). The nanowires obtained typically have a “bent” morphology (i.e., ratio of effective length to actual length of less than 1).

Other phage-based nanowires are prepared according to an analogous method.

Example 4 Exemplary Synthesis of Nanowires

Er(NO₃)₃.6H₂O (10.825 g) is added to 250 mL distilled water and stirred until all solids are dissolved. Concentrated ammonium hydroxide (4.885 mL) is added to this mixture and stirred for at least one hour resulting in a white gel. This mixture is transferred equally to 5 centrifuge tubes and centrifuged for at least 15 minutes. The supernatant is discarded and each pellet is rinsed with water, centrifuged for at least 15 minutes and the supernatant is again discarded.

The resulting pellets are all combined, suspended in distilled water (125 mL) and heated at 105° C. for 24 hours. The erbium hydroxide is isolated by centrifugation and suspended in ethanol (20 mL). The ethanol supernatant is concentrated and the product is dried at 65° C. until all ethanol is removed.

The erbium hydroxide nanowires prepared above are calcined by heating at 100° C. for 30 min., 400° C. for 4 hours and then 550° C. for 4 hours to obtain the Er₂O₃ nanowires. The nanowires obtained are substantially straight (i.e., ratio of effective length to actual length of about 1).

Other nanowires having different elemental compositions are prepared according to the above general procedure or other procedures known in the art.

Example 5 OCM Activity of Exemplary Catalysts

Exemplary catalysts selected catalysts from those presented in tables 1-20, were tested for their OCM performance parameters according to the general procedures above. In particular, the methane conversion and C2+ selectivities were measured at the lowest temperature required to obtain ˜>50% C2+ selectivity (condition A), and at the temperature which results in maximum C2+ selectivity (condition B). All catalysts under condition A showed C2+ selectivities and methane conversions greater than 50% and 15%, respectively, while providing C2+ selectivities greater than 55% and in most cases greater than 60%, while providing methane conversions greater than 18% and in most cases greater than 20%. It was noted that certain catalysts resulted in the almost total absence of reforming of methane to CO and H₂.

Example 6 Measurement of OCM Activity of Exemplary Catalysts

A flow reactor having an inner diameter of 4 mm is obtained from Altamira Instruments (Pittsburgh, Pa.). The reactor has a quartz tube that is loaded with 50-100 mg of catalyst to be tested. The reactor is run with 54 sccm (standard cubic centimeters per minute) of methane and 46 sccm of air. The temperature is varied over a range of 400-750° C. and gases are sent to a process gas chromatograph (GC) for analysis of all components. Methane is partially converted and the C₂ and C₃ selectivity and C₂ and C₃ yields are calculated from the raw GC data.

The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including U.S. Provisional Application No. 61/988,063, are incorporated herein by reference, in their entireties. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

LENGTHY TABLES The patent contains a lengthy table section. A copy of the table is available in electronic form from the USPTO web site (http://seqdata.uspto.gov/?pageRequest=docDetail&DocID=US09956544B2). An electronic copy of the table will also be available from the USPTO upon request and payment of the fee set forth in 37 CFR 1.19(b)(3). 

The invention claimed is:
 1. A catalyst comprising a mixed oxide base material for the oxidative coupling of methane, the mixed oxide base material comprising lanthanide elements and a dopant combination selected from the group consisting of Sr/Sm, Sr/Gd, Sr/Dy, Sr/Er, Sr/Lu, Sr/Ba/B, Ba/B, Ba/Sr, Er/W, Sr/K, Ba/Ce, Ba/Hf, Ga/Mg, Mg/Er, Y/Ba, Sr/Ga/Mg, Sr/Y, Sr/B/Y, Ca/B, Sr/Al, Ba/W, B/W, Sr/Ba/W, Sr/W/B, Ba/W/B and Sr/Ba/WB, wherein the mixed oxide base material has the following formula (III): Ln1_(a)Ln2_(b)Ln3_(d)Ln4_(e)Ln5_(f)O_(c)  (III) wherein: Ln1, Ln2, Ln3, Ln4 and Ln5 are independently different lanthanide elements; O is oxygen; and a, b and c are each independently numbers greater than 0; and d, e and f are independently 0 or a number greater than 0, provided that one of the lanthanide elements is not lanthanum when another lanthanide element is neodymium.
 2. The catalyst of claim 1, wherein: a, b, c and d are each independently numbers greater than 0; and e and f are independently 0 or a number greater than
 0. 3. The catalyst of claim 1, wherein the catalyst is a nanowire catalyst.
 4. The catalyst of claim 1, further comprising an alkaline earth metal diluent.
 5. The catalyst of claim 1, wherein the catalyst exhibits a C₂+ selectivity of greater than 50% and a methane conversion of greater than 20% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.
 6. The catalyst of claim 1, in combination with a support material.
 7. A method for conversion of methane to C₂+ hydrocarbons, the method comprising contacting the catalyst of claim 1 with methane and an oxidant source.
 8. The catalyst of claim 1, wherein Ln1 is Gd.
 9. The catalyst of claim 1, wherein Ln2 is Nd.
 10. The catalyst of claim 1, wherein at least one lanthanide element is erbium (Er).
 11. The catalyst of claim 10, wherein the mixed oxide base material comprises a physical blend of Er, or an oxidized form thereof, and a further lanthanide element, or an oxidized form thereof.
 12. The catalyst of claim 1, further comprising a support, diluent, binder, or combinations thereof.
 13. The catalyst of claim 1, in the form of a pellet or extrudate.
 14. A catalyst comprising a mixed oxide base material for the oxidative coupling of methane, the mixed oxide base material comprising three different lanthanide elements and a dopant combination selected from the group consisting of Sr/Sm, Sr/Gd, Sr/Dy, Sr/Er, Sr/Lu, Sr/Ba/B, Ba/B, Ba/Sr, Er/W, Sr/K, Ba/Ce, Ba/Hf, Ga/Mg, Mg/Er, Y/Ba, Sr/Ga/Mg, Sr/Y, Sr/B/Y, Ca/B, Sr/Al, Ba/W, B/W, Sr/Ba/W, Sr/W/B, Ba/W/B and Sr/Ba/W/B, provided that one of the lanthanide elements is not lanthanum when another lanthanide element is neodymium.
 15. The catalyst of claim 14, wherein the catalyst is a nanowire catalyst.
 16. The catalyst of claim 14, further comprising an alkaline earth metal diluent.
 17. The catalyst of claim 14, wherein the catalyst exhibits a C₂+ selectivity of greater than 50% and a methane conversion of greater than 20% when the catalyst is employed as a heterogeneous catalyst in the oxidative coupling of methane at a temperature of 750° C. or less.
 18. The catalyst of claim 14, in combination with a support material.
 19. A method for conversion of methane to C₂+ hydrocarbons, the method comprising contacting the catalyst of claim 14 with methane and an oxidant source.
 20. The catalyst of claim 14, further comprising a support, diluent, binder, or combinations thereof.
 21. The catalyst of claim 14, in the form of a pellet or extrudate. 